Why do some bodies age differently than others? And how much control do we have over it? A dedicated runner who won his first marathon at age 39, biologist Bernd Heinrich looks deeply at the connections between metabolism, diet, aging, and exercise. Picking up where he left off in 2001's Why We Run, Heinrich examines how our bodies change while we age but also how we can work with, if not overcome, many of these changes—and what all this tells us about the mechanisms of life, health, and happiness. The 80-year-old Heinrich also shares his considerations as he embarks on his final lap: a 50-kilometer race.