In recent years, the half-marathon has become the hottest distance in road running. It’s still not the most popular – the 5K continues to hold that position. But the half-marathon, or 13.1 miles, has enjoyed the most explosive growth, quadrupling in participants since 2000. In 2013, two million runners, 61 percent of them women, completed half-marathons in the United States. All told, 34 different half-marathons attracted at least 10,000 runners each.