A couple of weeks ago, Pennella ran Run Amuck at Marine Corps Base Quantico in Virginia — a 3 1/2-mile running race nothing like your neighborhood 5K fundraiser, he says.

"The race was ridiculously fun," he says.

"First, they hit you with a ladder hose from a Quantico Fire Department truck. Then on the first hill, a Marine makes you stop and do 10 jumping jacks and he makes you count — he had a bullhorn. Then you had to jump over bales of hay and run through a mud pit. And that's all in the first mile," says Pennella, 40, of his fourth mud run in the past three years.

Pennella is among a growing number of runners craving runs more offbeat or extreme than the typical weekend footrace, says Runner's World running expert and writer Bart Yasso, a longtime distance runner who spends almost every weekend at a race. He spent the weekend of Aug. 21 in Colorado.

"People are looking for the next challenge," he says. "I just saw Pike's Peak. They do a half-marathon distance ascent Saturday — 2,000 people run to the top of the mountain — and on Sunday the marathon is run, up and down."