Back in 2005 when I was thinking of joining Team In Training, I had a boss who was an avid runner. He ran a marathon in a ridiculously fast time, and he told me, “Don’t do a marathon. It’s too hard. Run a half marathon instead. It’s all the glory of a marathon, but none of the torture of training for a full.”

I took his advice to heart. What did I know? Back then, I was out of shape and thirty pounds heavier. Of course I was going to listen to the experienced athlete. So for the next three yea...