If, for some reason, a 26.2-mile marathon isn't quite difficult enough for you, perhaps you'd consider one that races up and down Pikes Peak instead. Registration for this year's Pikes Peak Marathon filled up in five hours, a sign of the growing popularity of trail races, which are arguably even more difficult than their road-race counterparts. Trail races offer fresh obstacles—uneven footing, forking paths, stream crossings—and their hills engage the back and chest while tearing at calves and thighs.
It makes running one no easy feat: Because of the steep ascents and the thin air present in particularly high climbs, prepare to run for a long time. According to the Pikes Peak race website, most runners clock 30-plus-minute miles about the treeline and look like a "stream of zombies...from Dawn of the Dead" as they near the summit of 14,110 feet. But in spite of the pain, these runs are gaining popularity: The number of trail races has exploded from eight in 1997 to 220 today, and sales of trail running shoes are up, the Wall Street Journal reports.