From the time he was a teenager, Justin Burger spent most of his free time, 20 hours a week or more, training for endurance races—ultramarathons, 24-hour mountain bike competitions, 12-hour kayak races. His hard work paid off. The 23-year-old Houston-based personal trainer routinely finished near the top of the podium at premier adventure races across Texas. "After I started college, though, I just had less time to exercise," he says, "so I expected the worst." Conventional fitness wisdom told him his results would plummet when his workout time was halved. Instead, he got faster.

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