By Laura Vanderkam
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. More
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