An award-winning, much-loved biologist turns his gaze on himself, using his long-distance running to illuminate the changes to a human body over a lifetime. Part memoir, part scientific investigation, Racing the Clock is the book biologist and natural historian Bernd Heinrich has been waiting his entire life to write. A dedicated and accomplished marathon (and ultra-marathon) runner who won his first marathon at age thirty-nine, Heinrich looks deeply at running, aging, and the body, exploring the unresolved relationship between metabolism, diet, exercise, and age. Why do some bodies age differently than others? How much control do we have over that process and what effect, if any, does being active have? 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 all this tells us about evolution and the mechanisms of life, health, and happiness. Racing the Clockoffers 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.