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...