Serial entrepreneur Joe De Sena and his business partner, Andy Weinberg, were on a 12-hour nighttime snowshoe trek in Vermont a decade ago talking about how people in general had become soft.

Of course they thought so. Weinberg had formerly been a Chicago swim coach infamous for pushing swimmers to spend at least five hours a day in the pool. And De Sena, well, he's got at least 100 running races under his belt, many of them 100 miles long.

Together, they came up with a big idea: a sporting event that catered to people who were willing to push themselves to the extreme. Who would be interested in something like that? A lot of people, actually.

Death Race

Imagine a race in which participants don't know the length--it could be 26 or 260 miles, two days or nine--nor do they have any idea what to expect. They might have to chop wood, chainsaw a hole in a frozen pond, climb a mountain, move a pile of boulders, or hit a target with only one bullet (and receive a four-hour penalty hike for failing).

In 2004, De Sena, Weinberg, and two others laid out and completed one such grueling race. The next year, 11 people registered for what was touted as "the Death Race," but only three finished. "Most of the men fell down and started crying. It was awful," De Sena says. "And The New York Times came and picked up the story, and then it just took off."

Today, Death Race attracts 350 people to each of four events held in Vermont and other locales around the world. The next event is planned for Mexico City.

Spartan Race

Not everybody wants to do a race that could kill him or her. So De Sena and Weinberg developed a still-hardcore but more structured alternative called Spartan Race.

It pulls in more than $50 million a year, De Sena tells me, from participants who pay $65 to $200 per event to climb ropes, crawl under barbed wire, and jump over fire, as well as a host of other boot-camp-like maneuvers. In 2014, Spartan Race will host 130 events in 17 countries, which is right up there with Ironman at around 120 events scheduled this year.

De Sena differentiates Spartan Race--which he says has evolved into a new sport called obstacle racing--from mud runs that are more focused on having fun and drinking beer. His goal? To get obstacle racing into the Olympics. The sport needs to be in at least 42 countries first.

"Most people...their coffee is cold, their car doesn't start, the dog barks, and they get upset over those things, and we think the reason is because they don't have any obstacle immunity; they haven't had to deal with really tough situations," he says. "Going out and doing one of these events [and] being completely out of breath, maybe being a little bloody and just completely out of your element and out of your comfort zone, helps build immunity toward the less difficult obstacles in life. It sounds silly, but we've gotten hundreds of thousands of emails about this."

Ironically, in building the Spartan Race business and obstacle-racing industry, De Sena says he's more sedentary than ever, taking phone calls and replying to emails all day.

"A lot of these emails are, 'Hey, you changed my life. I was 270 pounds [and now] I'm down to 190. This is exciting; I'm a Spartan,'" he says. "And so that keeps us going every day, because it just feels like we're doing something much bigger than ourselves."

If the idea of pushing yourself to your limits resonates with you, check out De Sena's new book, Spartan Up!: A Take-No-Prisoners Guide to Overcoming Obstacles and Achieving Peak Performance in Life, which comes out May 13 and is available for preorder on Amazon.