The Boston Marathon is the world’s oldest annual marathon. When it was first held in 1897 only 18 runners took part. Last year, however, more than 21,000 runners finished the 26.2 mile course. When did it become relatively normal for tens of thousands of people to together run through major cities anyway? Cameron Stracher, is the author of “Kings of the Road: How Frank Shorter, Bill Rodgers, and Alberto Salazar Made Running Go Boom,” a new history on running’s rise.He joins me now to discuss the history of the marathon and how the marathon — and the Boston Marathon in particular — became such an all-American event in recent decades.