A dedicated and accomplished distance runner who won his first marathon at age 39, Heinrich now turns his attention to look deeply at running, aging, and the body, revealing the convergence of influences on our bodies over a lifetime and exploring the unresolved relationship between metabolism, diet, exercise, and age. Why do some bodies age differently than others? Bringing to bear research from his entire career, and in the spirit of his classic Why We Run, Heinrich probes the questions of how we use energy and continue to adapt to our mutable surroundings and circumstances. Beyond that, he examines how our bodies change while we age but also how we can work with, if not overcome, many of these changes—and what this all tells us about evolution and the mechanisms of life, of health, and of happiness. As revealed through Heinrich’s observations of himself as a runner, Racing the Clock offers fascinating and surprising conclusions, all while bringing the reader along on Heinrich’s compelling journey to what he says will be his final race—a fifty-kilometer race at age eighty.