SHE WINCED DAILY at evidence of his affair, apparent now for some time. Which didn't make it any easier to take. Every time she walked into the bathroom, it reeked of his passion: sweat-soaked T-shirt on the floor, smelly socks stuffed into muddy Adidas. Often, she would rage silently over such scenes and move into the living room, only to find him plopped on a couch, exhausted (as usual), sipping a beer, unrepentant. Didn't he realize something grievous was happening - HAD already happened, in fact - to their marriage?

She complained. He shrugged agreement, kept stepping out for hours on end, came home too tired to talk, disinterested in dinner or dalliance, always lacked time for even a movie or the kid.

Only his passion mattered - running 80, 90, 100 miles a week. Obsessed with qualifying for the Olympic trials, he imposed his rigid training schedule on their life.Spontaneity died. She felt abandoned, unimportant, especially when he left her home weekends to trundle off to race. He skipped Thanksgiving dinner with her family: He had to run a marathon.

Finally, in the spring of 1975, having endured so many miles, she made her decision. He walked in the door, mildly elated from a second-place finish in the 10-mile Cherry Blossom Classic in Potomac Park, and good news met bad: She was ending their six-year marriage. She split that very afternoon; he took custody of a son.

"My wife stood by me as much as anyone would have under those conditions," says a regretful Steve Mahieu, 30. "But she knew I felt running was more important than my own family. I don't blame her; I blame myself. She'd say, 'Hey, we're having problems and a lot have to do with your running. I don't get to see you. We don't do anything together . . .' I'd agree and say, 'Yes, yes, yes . . ." but I kept on."

"There was always a race interfering with our lives," says his ex-wife, Barbara Verghese, also 30. She has since remarried - a Baltimore psychologist. He limits his running to two miles every other day.

THE JOYS and benefits of running are touted daily. Running, we are told, is both physical and spiritual, social and solitary. Cheap. All things to all people. Wonderful, in effect.

Pollsters Gallup proclaim 25 million Americans, 11 percent of the population, regularly lace up for the flight of foot. Advertisers love those runners - they're upwardly mobile, easy to target. They spend money on anything healthy, spawning endless commercial ventures, books and magazines, all devoted to carving up the spoils of trend. Runners are kicking up a bull market.

If happiness is . . . running, true happiness may well be running together.

Most runners take pride in the prowess of their mate. The power of winged feet skipping merrily along (together) seems so manifestly positive, so good for everyone. How can there be any doubts? No statistics hint at any cresting wave of "running divorces." On the surface, everything seems rosy.

Yet, behind the scenes, episodes of conflict are being played out between the runner and the spouse. Interviews with more than two dozen serious runners suggest that obsessive devotion to the sport can produce alarming upheaval in relationships.

Distance running demands all the time, dedication and energy required by any affair of the heart. And, as with any affair, exhilaration and risk go hand in hand. And running is risky, say spouses who have come to regard the sport as dangerously seductive. Hark: A wail of mourning rises from across the Sea of Domestic Tranquility.

Among the disgruntled are non-running wives who resent the hours running steals from a marriage. They can't plan meals; they gag on health food, the runner's staple. They're tired of schlepping off to the races, or staying home alone, nursing pulled hamstrings and sprained egos. Some charge their men with being lazy, shiftless, no account in matters of career: Only running matters.

The women don't understand, groan the men, pouring out their souls to strangers as they transcend the mundane on fraternal 15-mile workouts here and about. Come race day, if you listen closely at the starting line, you can hear the excuses in progress. "Aw, Joe, I'd be so much faster if only that dang woman would give me more time to run."

"It's a popular theme," admits Mark Johnson, 25, a single marathon runner (2 1/2 hours) with the D.C. Runners Club. "Running can be grounds for conflict unless the wife or girlfriend is understanding, or is into it herself . . . What a lot of people don't understand is that runners have their own priorities. I try to fit my job, my social life, everything around running, and I'm not an unusual case. If a woman gets in the way of my running, I get rid of her."

Johnson works in a Baltimore running boutique. His schedule is flexible enough to let him run when he wants and assemble a book on - what else? - running. Naturally, he hopes to marry a woman who runs. Running pals have even taken to scouting womens' races for likely prospects. "Most runners are looking for someone who will go right along with their life. I know of so many relationships that wouldn't have spoiled if that had happened."

BUT FINDING a running mate does not always prove to be the answer. A recent survey by a Rosslyn-based monthly, Running Times, suggests that the "fair number" of their readers receive domestic complaints about the amount of time running takes away from their life. But editor Ed Ayres says, "Relationships can also falter because the man and woman don't share a difficult endeavor, like running." A number of respondents claimed running improved their sex life.

Even with couples who share running, there is sometimes a clash over whether to run together or apart. Too much togetherness can make the faster runner feel bridled, held back. Too little togetherness, and the slower runner may come to feel abandoned. What to do?

Some women who run with faster men say they don't like being a burden, and voluntarily limit tag-alongs. Others say they love/hate being pushed to their limits, and insist on companionship for the challenge. Some go separate ways during the week - the work schedules dictating the when, where, how often and with whom - and make mutual accomodations on weekends. And others just don't make it at all.

Bobbi Conlan-Moore, 31, a writer-editor for Time-Life Books in Alexandria, first started jogging when she married Olympic marathoner Kenny Moore. She went from puffing three miles to zipping 26 herself. Running, she says, "became an integral part of my life, like eating breakfast." Her husband was supportive, "but his training . . . his running always came first."

They lived in Eugene, Oregon, a small town full of fellow runners, writers, artists. She ran; she wrote. He ran; he wrote about running. Most friends were recreational runners, or ex-runners or competetion runners. For eight years, she was known as "Kenny Moore's wife," she says.Bobbi Conlan-Moore came to feel like a "nonentity"; she got a divorce. She moved where the work suited her.

"I just found that world a little claustrophobic," she says.

MANY COUPLES SEEM to manage to jog around such permanent pitfalls as divorce and work out their running relationships. Still, bounding over hill and dale together isn't always a snap, as Rhyne and Jeff Lipsey are finding out. Jeff, a former high-school track coach, is an electrical engineer. He lives in McLean with his wife, Rhyne, a librarian, and daughter, Anna. He runs eight, maybe 10 miles every day after work, which usually puts him through the front door by 7:30. He doesn't feel like eating for a couple of hours, so Rhyne often eats early, with Anna. "It's a strain," she says. "Sometimes I resent it."

He has tried to compromise by running before breakfast, but it's just too painful. "He can barely walk in the morning," she says. So he gets up at 6 to fix breakfast for Anna. That's when Rhyne runs, unless, of course, Jeff has an early meeting, and can't wait around.

"We've had our altercations over running," she says. "Whenever I get mad, he'll say, 'I'll quit running. That's what I'll do. I'll quit. Is that what you want?"

"But I don't want him to stop. His family has a history of heart trouble. It's important for him to have regular exercise. He's a bear to live with when he doesn't run. It burns up his nervous energy, it relaxes him. And running takes less time and money than tennis.

"But when he talks seriously about competing, that's where I draw the line. People who train for marathons just don't have any family life. That business of going off to race every weekend is just too much. If he insists - well, that's going to be his little red wagon."

THE GRUMBLERS are not just wives. Some men resent women for treading on their traditional turf, and sometimes - horrors - even running faster. Or running at all. The first woman to crash the once-all-male Boston Marathon was treated none too kindly, and, for a time, Al Corfiled of Alexandria, a retired U.S. Treasury agent, gave his wife, Helena, the cold shoulder for running. He doesn't run.

"It wasn't that he said, 'No, you can't go.' But it would be very unpleasant around the house if I did run. He never said, 'Hey, have a good time,' or asked how I had done after a race . . . Just to get out the damn door was sometimes more than I could do," she says.

The crise de pied agitated their oldest son, now six. He was forever clutching at her, begging her not to run away."Mom, are you going to run tomorrow?" he would ask at bedtime. Frequently, he snoozed in the hall, trying to catch her pre-dawn exit. She tiptoed over the body.

"It was a nightmare," says Helena Corfield. "I felt guilty. But I wasn't depriving anybody. I was doing my job around the house . . .

"I guess Al resented the time I was putting' into it, but I resented not being able to do more. He's much more accepting now. He knows I really want to run and that I'm going to do it."

Helena Corfield started running about two years ago with a neighbor, Henley Roughton, 34, a marathon runner who founded the area's largest competetive running club for women, RunHers. Roughton's husband, Fred, a commercial airline pilot and marathon runner himself, coaches the club - as well as their daughter, Robin, 16, a high-school track star. The Roughtons represent the Contented Running Couple, the apparent majority among trundling duos, and Henley Roughton credits running with erasing life's self-doubts.

"It made me more content with my role as a housewife," she says."I'm not just a cook and washer of clothes anymore. I'm an accomplished marathon runner. I'm not the shrinking vine I used to be. That can create conflicts, but if you're happy with yourself, it's better for everybody."

Other women say the joy of distance running shakes them away from familiar notions and habits of what marriage should be, makes them feel independent, self-confident. Such mellowing doesn't always sit well with the spouse.

"'Running is going to come before everything else,' say a lot of women in the club,"' explains member - marathoner Ellen Wessel, 27, who designs and markets a line of women's running clothes. "It's a real shock to the husband who realizes mom isn't going to be around all the time. For a woman, there's a lot of guilt about hurting the marriage, and, in many cases, it does hurt the marriage."

IF IT WEREN'T running, it would be something else," says Dr. Glenn Schwarcz, a Washington psychiatrist and marathon runner. Like tennis or kayaking or working or meditating or drinking, running can become an obsession. Almost like some new-fangled pop religion that makes devotees look and feel so good, running has created the Runner-holic.

But the problem, says Schwarcz, is not the pastime so much as the personality type that may be attracted to it.

"The so-called 'Type A,' driven personality," he says, "is basically self-centered. That's why they accomplish things. They increase their self-esteem through accomplishment . . . It's the basic self-centered personality who tends to get off on a narcissistic glory trip.

"Running doesn't cause problems; personalities cause problems."

In fact, the doctor finds running useful for clients as an emotional warm-up exercise to therapy. A good run, he says, seems to help loosen up the internal logiams and just plain makes it easier to talk.

Schwarcz would like to attend to the psyches of runners, for which he feels especially qualified, but, alas, "Most runners just don't go to psychiatrists. Running itself is so' supportive, such a therapeutic experience, they don't seek out help."

He pooh-poohs the excessive time element some spouses cite as cause for friction.

"The quality of marriage is primary," he says. "You have a marriage that is basically not good and the spouses tend to become less and less involved with one another, spend more time by themselves. Running is an excellent opportunity to spend a lot of time away from a partner. What would appear on the surface - one spouse rebelling because the other spends so much time running - is underneath probably unstable, rocky. One is running to get away from the other . . . With or without running, the problem would probably come out sooner or later."

AFTER THE DIVORCE, Steve Mahieu moved to Albuquerque, where he manages a swimming-pool company. On the phone, he sounds wistful. He has seen the light, he says, come to terms with his obsession. He still runs, but is no longer driven to compete. He has changed. Yet, things can never be the way they were, and for this, he is sorry.

"The more runners I talk to, I find that my situation is not unique," he says. "A lot of marriages are staying together for the children, when the best thing would be to separate. It takes a shock like a divorce to really change you, to make you realize how you can let life get away from you . . .

"Runners should be aware of the danger signs. When life starts revolving around a running schedule and God help anybody who gets in your way, well, you're headed for trouble."