Nearing 60 years old and looking to shed a few pounds, Carey Rogers decided it was time to get more active. His achy knees ruled out basketball, and he never liked to run. So he spent a bunch of money on a bike, joined a cycling group in his hometown of Nashville, and off he rode.

Or rather, off they rode. Mr. Rogers fell so far behind the herd of Lycra-clad speedsters that he turned around and headed home. "I think I got lost on the way back," he said.

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