As I walked up to the crowd of 400 runners on the sweltering SoHo street, the nervous buzz felt like the first day of school: the happy sound of friends reuniting after the offseason, the chatter of newcomers connecting. It was the first day of the NoName Program, a marathon training regimen "powered by Lululemon" in which New Yorkers from every walk of life spend 16 weeks preparing for the absurdity of running 26.2 miles. We were far from alone. By many measures, America now runs on running.

In the sport's new golden age, more people are running than ever, and more records are falling faster than ever. Advances in technologies like carbon-plated shoes have allowed marathoners to reach times that would once have felt superhuman. But as elite runners are reaching new heights, many average

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