Robin Harvie was a fairly ordinary runner. He ran his first marathon after a bet. Then he found that although he couldn’t run fast, he could run long distances — very long. A casual hobby turned into a 120-miles-a-week obsession, and a training route along the River Thames morphed into a promise to himself that he would tackle the oldest and toughest footrace on earth: the Spartathlon from Athens to Sparta. This race, a recreation of Pheidippides’s legendary journey, is 150 miles long, crosses two mountain ranges, and is the toughest race on the ultradistance runner’s calendar. It isn’t at all ordinary. Harvie’s experience — from the mundanity of daily training routes to the extreme tests of the desert’s scorching heat and the darkest hours of the night — reveals the profoundly intoxicating experience of running, and the ways in which every mile taken is both a step further into the unknown and a pace deeper into the self.