When Hoby Darling, the CEO of Skullcandy Inc., was training for the Ironman 70.3 Hawaii, he thought he was the epitome of fitness. But after the 2009 race, which consisted of a 1.2-mile swim, a 56-mile bike ride, and a 13.1-mile run, Mr. Darling realized being fit for endurance races wasn't the same as being fit overall.

The 38-year-old says his focus on endurance training—he has competed in more than a dozen marathons and many Olympic distance triathlons since the early 1990s—meant he was ignoring strength, balance and flexibility. "I began to think about how many pull-ups I could do: one or two," says Mr. Darling.

When he returned from Hawaii, he typed "world's hardest workout" into an Internet search engine. The film "300" popped up and Mr. Darling read in a news article how the film's star, Gerard Butler, worked out "to get so huge for the film," he says.

Mr. Darling, who was an executive at Volcom, a California skate, snow and surfboard apparel company at the time, searched for a similar workout and stumbled upon a CrossFit studio in Costa Mesa, Calif. CrossFit incorporates weight lifting, body-weight strength exercises and calisthenics done in rapid succession.

"What hooked me is that it was harder than anything I'd ever done," he says. "I was sore for a week."