Jack Kaenel won the Preakness at 16. Ronnie Franklin won two of the three Triple Crown races at 17.

At 18, Mary Russ was working at a department store in Tampa, Fla., munching candy and gaining weight. She was closing in on 135 pounds.

At 28, Russ is the only woman to finish a major meet as the leading rider, the only woman in North America to win two Grade 1 stakes races and the only woman whose mounts have won $1 million in one season.

She didn't step inside a race track until she was in her 20s. At the time, "I just wanted a job," she says. She got it, and for the next two years she galloped horses and rode lead ponies, getting enough money to make ends meet by moonlighting in a grocery store deli at night. For recreation, and "to know the movement," she rode bucking bulls. "It was something different," she says now, sprawled on the floor of the jockeys room at Belmont Park.

It has been three years since a broken collarbone and a collapsed lung ended Russ' bull-riding days. Now, she is riding into the sunrise and sweating out her day in the sun. In five years, she has taken five days off. She has also taken her profession by storm.

Unlike most jockeys, Mary Russ didn't grow up in public. But she's fighting hard to stay there.

Horses--if not racing--were always a part of Russ' life. As a child, she rode with friends and at a stable, where a spirited horse was one "who ran fast going back to the barn." But when you are 5-feet-4 and weigh more than 130 pounds, nobody pushes you toward a riding career.

So Russ pushed herself. A friend had worked at a horse farm in Ocala, Fla., for a summer and suggested Russ try it. "I never came back," she says.

Just getting there was hard. "I was asked how much I weighed and I said 125 because that was the limit," she says. "He said, 'Are you sure about that?' I said, 'Yes sir.' I knew it was going to come off."

She has lost contact with a friend from high school, because when the woman asked her to be in her wedding party, Russ refused. "I know it was crazy," she says, but she was afraid to take time off from the farm. She can't say why, or what she was working toward. But she can say that she loved every minute of the five years she spent in Ocala.

"At the farm, it's breeding horses, breaking them, babies, little green horses," she says. "I'd hear some talk about $30,000 claimers, $50,000 claimers, and I didn't know what they meant. I thought, 'What is this? How do you know how much that horse is worth?'

"I just didn't have the slightest idea what it was all about, but I wanted to learn, because even if you stay on the farm and make a career of it, you should know about racing."

She found out at Calder Race Course in Miami, where she went barn to barn in search of work. "I wanted to get better," she says. "As far as being on a horse's back, I loved that, but I wanted to learn the whole business . . . whatever happened, I just wanted a job, that was all."

She got a way of life -- galloping horses in the morning, ponying in the afternoon, catching a nap, and off to the deli at night. She slept when she could, including "behind the wheel . . . I wrecked my car that way."

For an entire year, she had no time off. Not even a day. She was exhausted but not discouraged because by now she thought she knew what was coming next: diet and try it. "I got down to 115 on the farm and I thought that was light," she says. "I didn't know what light was."

Once she started riding, she learned. Now, she avoids liquids and worries about weigh-ins. She is 100 pounds and, although weight battles seem far behind, the war never is completely won. "It's such a fine line," she says.

Russ has been walking fine lines endlessly for years now. Because the memory span is so short in thoroughbred racing, she can't imagine taking off.

Love and fear are the motivating factors in her schedule. She loves riding, loves the work, loves the feel of a horse charging down the stretch. She fears that if she misses even a day, she will be replaced.

"If you take a week off, the horses you've been riding, someone else is going to ride them," she says. "And if they run good, they've got the mount."

This summer, her oldest sister was married in Tampa. Russ wanted to be there, but, "I was supposed to ride in one race that day, and, up here, I can't really be giving away mounts. But the horse scratched, so I got there after all."

When Russ was riding in Florida, especially after she finished the Tropical-at-Calder meet as the leading jockey (Jacinto Vasquez was No. 2), she was in demand, riding as many as seven races daily. "She rides as good as anyone else," Vasquez, who twice has won the Kentucky Derby, told the Miami Herald at the time.

Since Russ moved to the New York circuit in the summer, mounts have not come so readily and she worries, even though she passed the $1 million mark for 1982 at Saratoga, with a third-place finish.

In New York she has ridden primarily for trainer Roger Laurin, son of Lucien Laurin, who trained Secretariat and was one of the first to give Russ a chance. Not everyone was eager to try an apprentice jockey who was 26 and was female.

"Here she is ponying when she is 20-something years old and she tells me she wants to be a rider," recalls Dave Feldman, an owner/trainer who, instead, offered her a job ponying. "Hell no, no way. Most good jockeys start out when they are 16-17."

And, as Feldman isn't shy about saying, most good jockeys are not women. He counts no more than a half-dozen top-rate ones in the nation and says of Russ, who has since ridden for him, "She's probably the best." What sets her apart? "She listens (to instructions) well . . . and she lost all that weight. She just overcame everything." He also mentions the fact that Russ has been able to compete with the best in New York.

She is learning not to be intimidated by the best. One day last month, she rode in two races at Belmont, winning and finishing third. She won by keeping Angel Cordero in the slop along the rail; she finished third when Cordero's original third-place finish was disallowed because he had bumped Russ on a turn.

Feldman gives her his ultimate compliment: "Many times when I watched a race, just sitting there not knowing who is who, you couldn't distinguish her from another jockey. She gets down in the saddle. She looks like some guy riding a horse."

Russ' life is not what you might expect of someone who has earned more than $100,000 this year. She lives in a small apartment upstairs from a laundromat and, of course, close to the track. Pressed to name a hobby, Russ comes up with fishing, although it is her fiance Ricky Tortora's passion rather than her own.

Tortora is an assistant trainer (his father Manny gave Russ her first mount) so he, too, lives within the closed circle of race tracks. Theirs is largely a dawn-to-dusk existence, leaving little time or energy for the outside world. Russ has ridden in a taxi exactly once, has been into Manhattan exactly twice.

Her schedule and life style are partly reflective of her profession and partly reflective of her past. Russ grew up in a no-frills middle-class household. Vacations meant camping, and dinner out meant a meal at a friend's house.

It was both an all-American childhood -- parochial school, head cheerleader, homecoming queen -- and a tragic one. One of her sisters was hospitalized with mental problems. Her father, a fireman, developed severe health problems and, when Russ was in high school, committed suicide.

Despite the attention she now is receiving, she has trouble believing anyone would care about these or other details of her private life.

She remembers incredulously a visit from her mother when she was riding in Miami. "They really rolled out the red carpet for her. I couldn't believe they would do that . . . maybe for my uncle (a bank president), but this was for me, because of me." CAPTION: Picture 1, Mary Russ is the only woman jockey whose mounts have won $1 million in a season. UPI; Picture 2, Jockey Mary Russ has taken five days off in five years. UPI