As the wife of an endurance athlete, Caren Waxman wakes up alone every morning, including holidays.

"Mother's Day really upset me," says the Rockleigh, N.J., mother of three, age 47, whose husband leaves before dawn each morning for hours of exercise. In May, he will wish her a happy Mother's Day from Utah, where he will compete in a triathlon.

"It's selfish," concedes her husband, Jordan Waxman, 46, a private-banking executive at Merrill Lynch and an Ironman triathlete. He says he leaves notes for his wife and children before leaving for morning workouts.

With exercise intruding ever-more frequently on intimacy, counselors are proposing a new wedding vow: For fitter or for fatter. "Exercise is getting more and more couples into my office," says Karen Gail Lewis, a Cincinnati marriage and family therapist.

Jordan Waxman riding his stationary bike. Bryan Derballa for The Wall Street Journal

Newlyweds have long recognized the risks of potential sickness, infidelity and ill fortune. But few foresee themselves becoming an exercise widow. After all, the idea that one's beloved will take the occasional jog sounds appealing—until two miles a day becomes 10 miles, not counting the 20-mile runs on weekends. "His dream of doing marathons happened just when I got pregnant with our third child," Stephanie Beagley of Colorado Springs says of her husband, Michael, a purchaser for the U.S. Olympic Committee. "Now we don't have tons of time with him."

The exercise widow often wakes to an empty bed—a sure sign of a morning workout—and may find dinner plans spoiled by a sudden avoidance of anything heavy before a night run. Hoping for an hour of television or catching-up before bedtime? Forget it: All that early-morning exercise takes its toll. Mr. Waxman arrives home from the office after his children, ages 11, 10 and 8, have eaten dinner, and he hits the sack before they do. "I'm out of gas by nine o'clock," Mr. Waxman says.

"A lot of wives in my position would have left," Ms. Waxman says.

Ms. Waxman and Jacob in the yard, while Mr. Waxman was training on snowshoes. Bryan Derballa for The Wall Street Journal

Commitment to a demanding training schedule cuts to the heart of the issues couples often find themselves fighting about—who does chores, who gets time for themselves and who decides where and how the family has fun.

The threat can go beyond time issues. If one partner gets a new, buff appearance and a new circle of buff acquaintances, romantic possibilities can open up—and give the other spouse good reason to feel insecure about his or her own physique.

Couples therapists agree that commitment weakens as alternatives increase. Dr. Lewis recalls a client who realized she was unhappy in her marriage after she lost weight, became athletic and found she was attractive to men other than her husband. "She said, 'I married him thinking I didn't have a choice, because I was so heavy,' " Dr. Lewis recalls. Therapists say many relationships are based on similar levels of attractiveness; a shift in the equation can destabilize a marriage.

The effect of extreme exercise on divorce rates isn't clear. Even if research showed a higher rate of discord in homes where just one spouse is an endurance athlete, exercise could be a consequence, rather than a cause.

Among endurance athletes, though, resentment on the part of spouses is a common topic. The phenomenon may develop into what Pete Simon, an Arizona psychologist, triathlon coach and blogger, calls "Divorce by Triathlon." "I often wonder how many lonely wives, husbands, children of triathletes are out there wondering when the insanity is going to end," he wrote.

Of course, the surest way for a marriage to accommodate an intense exercise regimen is for both spouses to engage in it. Married for five years now, Walt and Kendel Prescott met in 2004 at the start line of a marathon. Mrs. Prescott, now 50, has run 313 marathons; Mr. Prescott, 57, has run 287. Their joke is that he keeps trying to catch up. "Running is a great excuse for me to be with Kendel," says Mr. Prescott, an airline worker in Atlanta.

The explosive growth in marathons, triathlons and other endurance sports comes largely from midlife converts such as Mr. Waxman, the Ironman triathlete. He and his wife celebrated a half-dozen wedding anniversaries and produced three children before exercise came between them.

'I'm amazed at what she can do athletically,' says Gary Berkowitz, whose wife, Lois, runs 20 marathons a year. Fabrizio Costantini for the Wall Street Journal

His exercise regimen intensified about seven years ago, eventually hitting two hours each weekday and up to five or six hours each Saturday and Sunday. "It became a sore point," Mrs. Waxman recalls. "I had three young kids and no family nearby. I heard myself badgering him: 'Family is really important. You need to be a part of their lives today.' "

Last summer, Mrs. Waxman persuaded her parents and her husband's parents to join her in what she calls "a family intervention"—a flurry of letters to Mr. Waxman urging him to exercise less.

But Mr. Waxman stood his ground. In his view, his athletic ambition shouldn't have surprised his wife. It arose from the same qualities that drove him to obtain two law degrees, an MBA and his position at Merrill Lynch.

His gargantuan training hours last summer were aimed at a particularly elite goal—a swim across the English Channel, which he achieved in September. "The English Channel thing, hopefully my wife and kids see it as a little bit inspiring," he says.

Rather than avoid exercise herself—the tack of many spouses who can't keep up with extremist partners—Mrs. Waxman hired a personal trainer with whom she works out four times a week. "My husband and I are on the same page in terms of being health-conscious," she says.

And all along she has mixed messages of support with pleas for more family time. "I love my husband, and I'm happy he's passionate," she says. "A husband wants to come home to a wife who says, 'I hope you had a good workout.' "

As for Mr. Waxman, he honors certain rules: Dinner with his family every Friday night. A date with his wife every Saturday night. And as often as possible, he turns competitions into family trips. "I make sure there's enough vacation time with the family," he says.

Some couples accept vast differences in exercise habits in each other. Lois Berkowitz, a 20-marathon-a-year runner and vegetarian, is married to Gary, a meat-and-potatoes eater who doesn't exercise at all. He doesn't like to shovel snow, so she does it.

Until he recently got a diagnosis of cancer, which he is battling, Mr. Berkowitz, 61, used to smoke two packs of cigarettes a day, including inside their home near Detroit.

Mrs. Berkowitz, 62, treasures their decades-long marriage. "We like each other," she says. "We're both introverts. We love our home."

"She's a very interesting person," Mr. Berkowitz says. "We have a very good life. And I'm amazed at what she can do athletically."

Mr. Berkowitz often accompanies her to races to volunteer and cheer at the finish line. He helps edit a running newsletter, and he developed software that helps his wife track her running life. She gives him her marathon T-shirts. "He gets credit for doing the run," Ms. Berkowitz laughs.

Write to Kevin Helliker at kevin.helliker@wsj.com