When people talk about running to get into shape, the shape they have in mind bears little resemblance to Dave Alexander's. At 5'8" and 250 lbs., with a belly that would do Ralph Kramden proud, Alexander looks as if the most exercise he gets is between the couch and the fridge. But beneath his barrel chest there beats the heart of a triathlete—one who has completed 266 of 267 triathlons, including 30 in one grueling 30-week stretch in 1986. (In triathlons, participants run, swim and cycle substantial distances.) "I'm in better shape than most people on the street; I just don't look it," says Alexander, owner of a Phoenix fuel-storage terminal, whose typical weekly training involves cycling 200 miles, running 30 and swimming 5. "I'm a great bar bet."

It was on a dare from friends 15 years ago that Alexander, who never played sports as a child in Long Beach, Calif., entered his first triathlon. Though "I thought I was going to die," he hung on to finish—and was hooked. Observes two-time triathlete of the year Mike Pigg, 34, who has competed against Alexander: "He always is in a good mood."

Although Alexander, 53, has yet to shed any significant avoirdupois from his exertions and often finishes dead last, he and his wife of 30 years, Marilyn, have plenty to smile about. With a blood pressure of 110/70 and a resting heart rate in the 50s, Alexander "definitely is my fattest, fittest patient," says Dr. Craig Phelps, who as physician of the NBA's Phoenix Suns sees his share of big men. "Dave is one in a million."

Turkey thinks so and has made Alexander an honorary citizen. In Malaysia he enjoyed a private dinner with a sultan. "People all over the world know Crazy Dave," says Alexander, who has accumulated 600,000 frequent-flier miles. "But I would give up everything just to finish in the middle of the pack."

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VANISHED WITHOUT A TRACE

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After Jaycee Dugard's rescue, a look at the cases of six young people who went missing in 2009

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