FORTUNE — Legend has it that Pheidippides, the first person to run a marathon in ancient Greece, died as a result of the strain. Yet these days, thousands of seemingly rational people don’t view that grisly result as a reason not to run 26.2 miles or undertake other similarly lengthy events.
Indeed, the number of U.S. marathon finishers increased from just shy of 300,000 in 2000 to 525,000 in 2011, according to USA Marathon. Roughly 2.5 million people participated in a triathlon in 2011, a huge leap from just under 1.5 million in 2008, according to the Sporting Goods Manufacturers Association.
Curiously, many of these endurance athletes haven’t given themselves to sport at the expense of professional success. Races court sponsors by claiming that they cater to an upper-income demographic. In 2006, Mary Wittenberg, CEO of New York Road Runners (which stages the ING New York City Marathon) told the New York Times that her average runner’s household income was $130,000. USA Triathlon says the average triathlete’s household income is $126,000.
So what’s the appeal of endurance sports for people who are busy earning money, too?
First, such folks like difficult tasks. Gordo Byrn is a former finance type who tried to run three miles in 1994 and had to walk home. Unhappy that he was so out-of-shape, he started training for triathlons and finished the Ironman Canada in 2004 in 8 hours, 29 minutes, and 55 seconds — good enough for second place.
Byrn now coaches other triathletes and says that “almost everybody has full-time jobs and I’d say more than half have kids on top of it. I think [for] a lot of folks, what appeals to them is this challenge of trying to balance training with family and work.” He says that “the whole craziness of it appeals to people” in a way that simply running four miles a few times per week for health and fitness doesn’t do.
Professionally successful sorts “have got great habits that fit an endurance framework,” says Charlie Brown, a sports psychologist who works with both professional athletes and executives. Folks who were not necessarily on the jock side of the geek-jock divide in high school may be off the charts in terms of persistence — the major characteristic needed for marathons and long triathlons. “You don’t have to have the hand-eye coordination you need for tennis,” he says — or the height and dexterity you’d need for basketball, volleyball, etc. “You’ve just got to be able to grind it out and endure.” People who are successful in business often know how to set big goals and break them down into doable steps.
The relationship between professional and sports success may go beyond perseverance, though. Robin Kanarek, dean of Tufts University’s Friedman School of Nutrition Science and Policy — and herself a triathlete who frequently wins her age group (she’s 66) — conducted an experiment in which she gave people short descriptions of individuals. The major difference between the descriptions was how much people exercised. Research participants described the intense exercisers as “more motivated, more dedicated,” she says — characteristics successful people may want to promote about themselves.
But Kanarek says what she found most interesting was that the less fit were deemed about five years older than the intense exercisers. “I think there’s that desire to feel younger,” says Kanarek, to perhaps stave off mortality.
Couple that with another of her research findings — namely that rats weaned off of intense exercise go through classic symptoms of withdrawal very similar to the kind experienced by rats given morphine — and you can start to see how an achievement-oriented sort might start with a half-marathon, but then decide she needs to do more. A marathon. An ultra-marathon. An Ironman.
“When people become really extreme, they do get into this addictive pattern,” says Kanarek. “If they’re running when it’s snowing, or doing ultras — running 100 miles through Death Valley — there’s something else going on there.”
The key to recognizing if one’s attitude toward endurance events is healthy or not, says Brown, is to ask “Do you have an identity in your life other than that as a performer? That’s essential.” While “resting on your laurels” has a negative connotation, in a society where many people are out-of-shape, being able to run a 10K — and earn six figures — is pretty good too.