Registration for this weekend's Pikes Peak Marathon filled up in five hours.

"It's become a stampede," says Ron Ilgen, race director for the marathon up and down Pikes Peak on Aug. 22 and the half-marathon up the mountain the day before. Combined, the races admit 2,600 runners.

From five-kilometer to 100-kilometer races, runners in growing numbers are veering off concrete onto dirt, grass, rock and gravel. Nationwide, the number of trail races has nearly doubled since 2004 to 220, up from only eight in 1997, the 14-year-old American Trail Running Association says.

A typical crossover is William Hinz, a 63-year-old lawyer in Orange County, Calif. A veteran of seven urban marathons, he recently discovered trail running and finds he likes the different challenges it presents. Take his eyes off the trail and he might stumble. Fail to look up, however, and he might go the wrong way at a fork in the path. During one recent race, "there was a plank you had to take across a stream," says Mr. Hinz, who is running the Pikes Peak Ascent half-marathon on Saturday. "I just hope to finish," he says.

Trail running provides a more comprehensive workout than flat-land running, because hill climbing engages the muscles of the abdomen, chest, back and shoulders. Apart from the torture it inflicts on calves, thighs and buttocks, trail training raises a question that rarely comes up ahead of flat-land races: How many pull-ups can you do?