What do you do when your body doesn’t respond to physical training in the same way it did just a few years ago? How do you react if you feel pain in your rib cage, yet X-rays show no fractures?

These are some of the questions I was asking myself in 2013. Early that year, I was training for a two-day, 150-mile cycling event, but it didn’t seem like I was getting any stronger.

Throughout the summer and fall, I’d also been...