When Mike Grollman flew to Chicago this October to compete in the Chicago Marathon, it was an anniversary of sorts. His very first marathon, back in 1992, was also in Chicago. “It’s been almost twenty years,” Grollman laughs. “And I’m still doing these things. There’s got to be some deep-rooted neurosis that I’m working through.”

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Since ’92, Grollman has competed in the Chicago Marathon at least 15 times. (They’ve started to blur together so he can’t be sure.) He’s also run marathons in London, New York, Boston, and Washington D.C., among others. “I’ve done them all,” he says. “It’s a sickness.” Preparing himself physically for each marathon requires months of training. “I probably average 65 to 70 miles per week when I’m gearing up for a marathon,” he says. And that takes time, which isn’t a luxury for the 45-year-old president of Bell-Lap Communications, an entertainment marketing firm based in Los Angeles. Grollman also has a wife and two sons who expect to see him occasionally.

How does he balance his work and home schedule with the exhausting and time-consuming training required to compete in a marathon? “I don’t,” he admits. “I end up apologizing a lot.”

There are 850 marathons in the U.S. alone, up from 300 in 2000, and five “World Marathon Majors,” including Berlin, Boston, Chicago, London and New York City. Around two million people sign up every year, and 487,000 of them crossed the finish line in 2012 (compared to 224,000 in 1990). The list of CEOs who compete in marathons is long—there’s Steve Burke of NBC Universal, Klaus Kleinfeld of Alcoa Inc., and Larry Zimpleman of Principal Financial Group, to name just a few—and their numbers are growing every day.

How do already overextended professionals moonlight as marathon runners? Hal Higdon, an 82-year-old fitness guru and marathon veteran (he finished fifth in the 1964 Boston Marathon), says it’s all about finding the right program. Twenty years ago, he designed one for his son, Kevin, an accountant at Peat Marwick International (now KPMG) who wanted to train for a marathon “while working for a demanding firm.” Higdon’s corporate worker-friendly program—with an emphasis on weekends and early morning weekday runs—became a best-selling book, Marathon: The Ultimate Training Guide. It’s still used by many working professionals today, including Illinois Bank of America president Tim Maloney, who Higdon claims followed the program while training for this year’s Chicago Marathon.

It’s not easy. On some mornings, you’re required to run ten miles before the rest of your company has had their first cup of coffee. “I’ve had people ask, ‘Can I do five miles in the morning and five miles during lunch?'” Higdon says. “My response is always, ‘Well sure, if you can find a marathon that lets you run 13 miles in the morning and 13 in the afternoon.'”

It may sound grueling, but Dr. Charlie Brown, a sports psychologist from Charlotte, North Carolina, says the additional hours and effort rarely faze an over-achieving entrepreneur. “They’re used to doing hard work and being uncomfortable,” he says. “That’s how they got to the position they’re in.” The problems arise, he says, when they don’t allow for recovery. “That’s how you build strength,” he says. “You don’t get stronger from the training, you get stronger when you recover from the effort.” They forget things like sleeping, or proper nutrition, which can make or break a marathon runner. “You have to learn the right language when working with a high achiever,” Dr. Brown says. “They won’t respond to requests like ‘You need to slow down.’ But they understand criticism like ‘Don’t be lazy with your recovery.'”

There are legitimate concerns about whether training for and running a marathon, which places considerable stress on the heart, is a good idea for somebody with an already stressful day job. A study published in November by the journal Heart claimed that “short bursts” of aerobic activity are perfectly healthy, but “running too fast, too far and for too many years may speed one’s progress towards the finish line of life.” Other studies aren’t quite so grim. The New England Journal of Medicine reported in 2012 that over the last ten years, nearly 11 million people ran long-distance races, and only 59 suffered from cardiac arrests. That breaks down to approximately one in every 259,000 runners. You have a better chance of drowning in a bathtub (one in 10,455) or dying at the office (one in 48,000) than dropping dead during a marathon.

For Grollman, training for a marathon is actually the opposite of stressful. “I’m never more peaceful and zen than when I’m out running,” he says. “The longer the run, the better. The stress just melts away.” The intense pre-marathon training, with its constant early mornings and double-digit mile counts, is part of the appeal for him. “I’d want to do that anyway” he says. “Marathons are how you rationalize that behavior. You tell somebody you’re getting up every morning to run 18 or 20 miles, they’ll think you’re a dope. But tell them you’re training for a marathon and they’re like ‘Oh, that’s cool.’ It’s an excuse so people don’t think we’re weird.”

For the Chicago Marathon, Grollman barely remembers his results. “I think I was 158th in my age group,” he says. “The winner was showered and on the plane back to Kenya by the time I crossed the finish line.” But he’ll be back in training tomorrow, for whatever marathon is next. “When you’re out there in the morning,” he says, “and it’s just you and the other freaks, that’s what it’s all about.”

Derrick Hesser, who’s competed in 10 marathons during his career, hasn’t run one since 2011. It’s partly because he just doesn’t have the time anymore. Hesser runs his own branding agency, Connascent, Inc, which comes with a hectic travel schedule, and he’s also a father to four kids. But he misses the physical demands of marathon training. “It sounds counter-intuitive, but the exercise makes you a lot more productive at work,” he says. “Since I stopped running marathons, I’m more sluggish, I have less get up and go. I’m still waking up at 4am every morning, but instead of running, now I’m getting on a plane to fly 400 miles to a work meeting.”

He’s thinking about training for another one soon. “I need my energy back.”

(This story originally appeared, in a slightly different form, in Bloomberg Businessweek.)