Born to Run--For 300 Miles

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Until his birthday conversion, Karnazes led a fairly conventional life. Raised in Southern California, he earned an M.B.A. and worked as a sales executive for GlaxoSmithKline. His family's California dream was shattered by the death of his younger sister in an auto accident. The event, Karnazes writes, shut off the family psychologically, pointing him inward. That happens to be a good direction for ultramarathoners. "You get to turn off your mind for long periods of time," says JoAnn Dahlkoetter, an ultramarathoner and sports psychologist at the Stanford University Medical Center. "It's a real form of therapy for a lot of people."

For Karnazes, ultramarathoning is no longer therapy, it's destiny. "I've pushed the envelope with regard to the pain threshold, the psychological elements that play into running those kinds of distances," he says. That kind of attitude will help him go a long, long way. --Reported by Andrea Sachs/New York