"Step after step for 26.2 miles, hundreds of thousands of people run marathons. But why--what compels people past pain, lost toenails, 5.30 am start times, The Wall? Sports writer Matt Fitzgerald set out to run eight marathons in eight weeks across the country to answer that question. At each race, he meets an array of runners, from first timers, to dad-daughter teams and spouses, to people who'd been running for decades, and asks them what keeps...