JULIE RIDGE is a Broadway actress who took off her clothes in "Oh! Calcutta!," left the show so she could swim the English Channel, then became the first person to swim twice around Manhattan, a distance of 56 miles in 21 hours 2 minutes and 49 seconds. She jumped into the East River at 10:20 p.m., came down the Hudson at 2 a.m., rounded the Battery just before dawn, then crossed under the Brooklyn Bridge in time for the sunrise. The debris in the water wasn't too bad, although she did say she encountered a few things she wouldn't have bottled and sent home to her mother.

She grew up in Arlington, and didn't even swim on her Yorktown High School team. Now, though, she's on to one of the ultimate tests of endurance: the Ironman Triathlon in Hawaii, an event that blurs the line between self-improvement and self-destruction, a swim-bike-run that is the oldest, toughest and perhaps most insane of them all. It is the fitness craze gone slightly mad, a race incomprehensible to the weekend jogger. Some triathletes finish by collapsing in the first-aid tent and offering up their veins for IVs. Most others describe a "morphine-like" high that can be understood only by those who have done it. During training, top competitors have to replace lost calories with the equivalent of 12 meals a day. No one has died, yet.

Today, Julie Ridge will wade waist-deep into the Pacific to begin a 2.4-mile swim, followed immediately by a 112-mile bike ride and then, just so things don't get dull, a 26.2-mile marathon. Several years ago, one woman literally crawled across the finish line. Another year, John Dunbar, a former Navy Seal, was reported by Sports Illustrated to have been so crazed at the end of the race that he was staggering into cars in the parking lot and accusing his support-van driver of trying to poison him. (Dunbar, a 30-year-old part-time diver in San Diego, now denies this, but does admit that his support crew ran out of water during the marathon -- so "I ended up having to drink beer the last 10 miles. Sort of brought me into a state of delirium.")

Ridge figures the race will take her 17 hours. She doesn't want to win, she just wants to finish. "I'm not a fast person at anything," she says, sunk comfortably into a deep couch in the study of her father's McLean condominium, not far from where she grew up. "But I last a long time."

Like other self-taught middle-of-the-pack athletes, Julie Ridge is driven by the concept of endurance, a woman tantalized less by her splits than by what she'll find by teetering on the edge of her own limitations. Still, there are less strenuous ways of exploring the soul. Why does someone who seems no more abnormal than the rest of us -- with the exception of her appearance in "Oh! Calcutta!" -- feel the need to do something so physically extreme?

Her mother, Jeanette Ridge, a doctoral candidate in genetics, thinks her daughter has always "wanted to know herself -- what an amazing thing to dig down into your own psyche like that." The attention hasn't hurt her acting career, either; when all this is over, Ridge would like to settle back into her Manhattan apartment and star in her own soap opera about life at a health club. There's also linkage: "Instead of saying, 'I can't,' " says Ridge, "you say, 'Well, I swam to France one day. I can get through this.' " Then, too, so few things in life can really be controlled -- in Ridge's case, she's had the usual troubles with love, acting auditions and her parents' divorce -- so setting a goal that can be met by sheer willpower has a seductive appeal.

"When I was coming down the Hudson River," she says, "I felt like I was riding a very powerful force. You could feel the current. It was carrying me, and its will as well. There's a sense of 'I do have some control, but I do have to listen to the elements and play the game that's being played here.' It's exciting -- to beat the waves, to say, 'You're bigger than I am, but I'm going to do it.' "

She is 28, stands 5 feet 3 inches tall, weighs 115, including two impressive biceps, and has dark eyes, dark hair and a taut, compact body. One sportswriter once described her as dreamy. When she's in training, she lives on enormous quantities of ice cream and whole-wheat lasagna, although she still has trouble keeping on weight. "Eating is no longer a really pleasant thing," she says. "It's just a matter of doing it whenever I think of it, whenever I get anywhere near a refrigerator." Her Gatorade comes free, compliments of the sponsor that is sending her to Hawaii, but she's still $20,000 in debt from her previous swims. Her father, a financial planner, helps out.

After her Channel swim, when she'd reached the shore of France, she flopped exhausted into the boat with her father. "I asked her how she felt," Frank Ridge recalls, "and she said, sort of wistfully, 'I'm tired, Dad, but I have some energy left. I still don't know how far I can go.' "

The Ironman might just let her know. The Swim

The swim begins at the Kailua Pier on the island of Hawaii, heads out for 1.2 miles in open ocean, then doubles back. The immediate problem is seasickness; a number of swimmers have become nauseated from the waves. Leg cramps attack others, as do the jellyfish and sea urchins. One year somebody was stung in the eye by a jellyfish, but he managed to finish with a patch over his eye. The first year, 1978, one guy barely knew how to swim, but in the early cowboy days of the triathlon, this was not considered a major problem. A lifeguard in one of the boats merely gave him pointers on his stroke as he inched pathetically through the swells.

The Ironman Triathlon was the 1977 baby of Capt. John Collins, then a 41-year-old Navy commander who fixed submarines at Pearl Harbor and who now says that it is untrue, as popular legend has it, that he was "drunk out of his mind" when he came up with the idea. This was at an awards banquet for an around-Oahu relay race, and sometime during the dinner at a fake Polynesian place the runners, swimmers and bikers who were there began arguing about who was in better shape. They never settled it. Instead, Collins and his friends decided to combine the Waikiki Roughwater Swim, an Oahu bicycle race and the Honolulu Marathon into one Gargantuan event, at which point Collins stood up and announced it right there.

Let's get this straight: Had he had something to drink?

"Oh, a little bit," he says.

For Julie Ridge, the swim will be something she can do in her sleep. In fact, she does doze off during her swims sometimes. She says she continues to stroke, just not as efficiently.

"During training," she says, bare feet tucked under her sundress, "there's a place after you've gotten warmed up, after you've gone on automatic, and you're in what has been referred to as the Twilight Zone or the Ozone. And then the imagination can do some wonderful things. It's really like traveling." She has a rich, whispery voice and the self-possession and self-absorption common to an actress. "It's a very special kind of warm, protective place. The only time I find that place is in the water . . .

"Most of the decisions I've made -- like the decision to swim the English Channel, the decision to swim twice around Manhattan, the decision to take an apartment, the important decisions in my life -- have all been made in the water. It's like something becomes clear, and it clicks . . .

"In acting, there's a moment where you are part of the scene with the players, and with the audience, when everything works. Something happens, something very special and magical. There aren't words to describe it. It's like the spirits join, or whatever. You get that in the water, a lot. Sometimes you get it in wonderful moments in bed with somebody, that sensation that you are not limited to what you are, physically. It's wonderful."

Ridge says she sees her swimming as positive, that she's not swimming away from something but toward something, although what part of herself that is seems as amorphous as the water itself. "There's something very wonderful and playful about the water," she says. "There's a sense of power, and there's also a sense of insignificance. Here's this little tiny thing in this huge, vast body. It's different than air; you move through it differently. It feels like being touched all the time."

She grew up on Abingdon Street, a "quiet little kid" who recalls she had a favorite place between her dresser and the wall where she would sit seriously and suck her thumb. "I was the kind of little kid that I hate now," she says. "I mean, I took a briefcase to the fifth grade." She says she used to play a game by getting out a volume of the encyclopedia, letting it fall open and reading whatever was there. "Maybe I wasn't very social, maybe I couldn't deal with people," she says. At Yorktown High School, she describes herself as someone who "never felt like a popular person. I wasn't a cheerleader type."

She did have a good time on stage. From the time she played Maria in "West Side Story" at summer camp, she'd wanted to be an actress. At Boston University, she majored in fine arts, but was selected for only one part in four years. Her first year there, her parents were divorced -- "a kind of going-off-to-college present," she says ruefully. She was swimming a daily mile at the university pool, but just, she says, for pleasure.

"I love the idea that people who do great things do so out of suffering," she says. "And I keep thinking, 'My story must be pretty boring,' because I grew up in a big house with two gorgeous collies and two parents who were wonderful to me, even though they didn't like each other too much for a while, and two sisters who I didn't want to slam against the wall too often. I had a pretty good childhood. I don't have any complaints. I'm not doing this out of some need to cover something."

So why?

She gets a little impatient.

"Just because," she says. "Why not?" The Bike Race

Staggering out of the waters of Kailua Bay, the Ironman triathletes switch to their bike shorts (the average changing time for the top 100 finishers is 90 seconds) and begin heading 56 miles through black lava fields and cattle country, then straight back. If the heat, loneliness and mean headwinds don't get them, the cattle guards will. They're big, nasty grates in the middle of the road, good for catching bike wheels and hurling the rider over the handlebars and into a rock. During last year's bike race, the overall winner, Dave Scott, avoided the cattle guards -- but not his own hypoglycemia and dehydration. Unable to focus, he veered off the road and struck a boulder. He needed 60 stitches in his head.

On top of that, the ride is ugly. Kay Rhead, the race director, describes the lava fields as looking like what happens when the peach pie oozes over and becomes a black, crusty mess on the bottom of your oven. Which prompts the inevitable question: Hawaii has some of the best scenery in the world. What would be so bad about staging the triathlon in an attractive spot?

"We don't call it the Marshmallow Man," says Rhead, primly. "The course was not chosen for esthetic value."

The bike ride is where things begin to get hairy. The first year, when there weren't any aid stations, Ironman founder John Collins got so hungry he stopped at a greasy spoon for chili. Chili. "It sat rather heavily on my stomach," he recalls.

Another competitor is alleged to have knocked on the front door of a house and asked for dinner. He is said to have been successful.

Other racers have taken off entire hours to eat, particularly in the early years when there was no time cutoff. (This year, everyone has to finish in 17 hours.) Many would stop at food stands that had been set up by triathlon sympathizers to eat sushi, rice balls (very popular), roast pig, or steak and lobster.

"Iron Stomach, I think it's called," says Rhead. Then there was the guy who actually went to Sears the night before the race, bought an unassembled bike, only to turn up at Collins' house at midnight and ask him to put it together. Collins refused on the grounds that he was too busy staying up all night himself -- making trophies, which were five-inch high oddities of nuts, bolts and copper tubing, each with a hole in the top. Like the course itself, ugly.

"I thought they were sort of cute," says Rhead.

The Ironman has become big time since that pioneer year when 12 people finished and three did not. "Back then, it was ridiculous," says John Dunbar, the guy who drank beer the last 10 miles of the run. "No one had ever done it. It wasn't just competing with another person, it was seeing whether a human being could do something like it. That was one of the factors the first competitors had to overcome -- that we might experience some medical disorder after nine or 10 hours on the course."

"It represents less than prudent living in my mind," says Dr. Samuel Fox, a Georgetown University cardiologist, "but everybody to their own deserts, as they say." So far, no one has experienced any medical disorder mysteriously brought about by 10 hours of vigorous exertion. But serious problems can occur during a triathlon, including, says Fox, dehydration, low blood pressure, a temporary poor kidney function and an irregular electrical activity in the heart. Last year, winner Dave Scott had to be given glucose before he could talk to the press. But with proper training, Fox says, the body seems able to handle the physical stress.

"I don't think there's any psychiatric kookiness about this," he adds. "I know quite a few people who seem to be very nice people, doing good professional work, who are involved in triathlons. It takes a lot of discipline. But they're not squirrelly or up a tree."

This year, there are expected to be 1,000 racers, including 150 women; 3,000 applied. The competitors are from 31 countries, and tend to be affluent, textbook Type A's. The men are generally in their late thirties. "They're looking for a new challenge," says Rhead. "The job doesn't do it for them like it did in their twenties." The women, on the other hand, tend to be in their midtwenties. Rhead surmises that this may because this is the first generation of women who had real sports training in high school and college.

Ridge herself is not a professional swimmer. In fact, when the BU swimming coach took one look at her stroke her sophomore year, he told her to forget it. Acting was more promising. After she graduated in 1978, she got a job with Oh! Calcutta! in Chicago, and when that show closed, moved to New York to appear with the Broadway company. ("It's something you can miss and you'd certainly get through life," she says, adding that the nudity never bothered her. "It is a funny, almost passe', light sexual comedy.") She spent her days swimming a mile or two or five at the health club at Manhattan Plaza, an apartment complex near Times Square. The Channel idea popped into her head during one of those swims.

In May 1982 she left Oh! Calcutta! to train full time, getting some coaching but mostly reading everything about distance swimming that she could get her water-wrinkled hands on. Then, at 5:10 a.m. on Sept. 10, 1982, Julie Ridge stepped off Dover's Shakespeare Beach and began swimming toward France. She was covered with 1 1/2 pounds of lanolin and Vaseline. Her father, her best friend and a pilot accompanied her in the boat. Conspicuously missing was the man she loved.

"I thought he was very important to me in my life, and I thought I was important to him in his," she says. "I somehow thought if he could see me do this wonderful thing, if he could see me be strong and powerful, then he would love me. He decided he couldn't go for various reasons. But if he had really wanted to be in England, there are no excuses. He really didn't want to be there, obviously. So when I got there, I realized I had to be doing it for another reason. It couldn't be to win his love, because obviously I hadn't. I was doing it for me, I was doing it to prove to myself that I really was this special, wonderful kid that my mommy and daddy always told me I was when I was growing up."

At 11:05 p.m., Ridge reached France. She had a banana, then began sending telegrams. The Marathon

The Ironman marathon is hell. Marathons are hell, anyway, but most runners at least spend the days before a big race sitting around and eating, not fighting jellyfish and 35 mile-an-hour headwinds. There are IVs available along the course, and Rhead says it always helps competitors to have one afterward. "They bounce back quicker," she says.

At this point, those in the middle of the pack are starving. "They'll eat their shoes," says Rhead. Instead, they stop at the official aid stations and consume nearly two tons of bananas alone. Also available are oranges, Gatorade, Pepsi, water, chocolate chip cookies and, in previous years, guava jelly sandwiches.

One year a runner was so out of it that he stopped at an aid station and grabbed what he thought was a wet sponge, only to discover when he wiped his face that it was a peanut butter sandwich.

The weekly training for the average participant is about seven miles of swimming, 230 miles of biking and 48 miles of running. The elite, of course, run more. Champion Dave Scott has been known to run 10 to 15 miles, then bike 75, followed by a three-mile swim -- every day.

Ridge doesn't do near that much, instead running 15 or 20 miles a week, biking 120 to 150 and swimming seven to 10. "My training is really unorthodox," she says, "and I would never prescribe it as a program for somebody. It's filtered down through experience, and through what I know is the minimum I can get through and not hurt myself. I may be misjudging this, but I'm not worried about finishing. It'll be a matter of how much I hurt when I'm done."

After it's all over, she knows there'll be an incredible, temporary high -- and then a fast depression. "Usually it takes a couple of hours for the depression to hit," she says. "It's a physical depression that brings on a mental depression. It gets cyclical. I was incredibly, severely depressed after the English Channel swim. It lasted, mentally, for weeks."

So that's when she set her goal on the Manhattan Double, as she called it. Already, her record's been broken by Stacy Chanin, a University of Maryland senior who completed a triple swim around the island on Aug. 29. Ridge, who had given her tips on swimming at night, was in her escort boat.

Her long-range goals after the Ironman?

"I want to swim until I die," says Ridge. "I want to stay healthy and strong. I want to have kids, eventually." She recently met an endurance swimmer in Mexico, "a very, very special person" who makes her think "it's definitely possible to have some of the things in life that I want." She is the coauthor of "Take It to the Limit," a book about endurance coming out in the spring. She'd like to write some more books, and maybe a movie.

"I still don't know what my limits are," she says. "I still don't know how long I could stay awake, or swim, or bicycle, or any of those things. It's fun finding out."