The question seemed innocuous enough, but it offended 55-year-old Dan Brenden.

"They asked me who my biggest competition is," said the Maricopa County attorney. "They don't understand what ultra running is. We all want everyone to accomplish their dreams. You just put everything you've got into it, and you're testing yourself; you're doing the very best you can."

Every athlete who ventures into ultra running has different reasons for doing so. For Catra Corbett, it was a way to beat an addiction to drugs. For Karl Meltzer, it seemed a natural progression from marathons to endurance races. For Diane Van Deren, it was a way to continue to make money as a professional athlete, even after three children, 10 years of seizures and brain surgery.

"It's the challenge thing," said Meltzer, 38, who won the Wasatch 100 last weekend and owns the course record. He's currently pursuing another record in the Rocky Mountain Grand Slam. "As you get older, you get slower; so you just go farther."

Ultra running doesn't seem like a sport with mass appeal, but the sport is growing in popularity, and there are nearly three dozen 100-mile races in the United States alone.

Brenden's finish in the Wasatch 100 last weekend was his ninth finish this year. He can't even count how many ultra races he's competed in, but he does know what his record is.

"I've never dropped out of a race in my life," Brenden said. "I have a mental edge in the rest of my life now. (Ultra running) becomes part of your life; it shapes your life."

Most ultra runners get emotional when talking about why they push their minds and bodies past the point of all reason. Often, they say, they can't find the words to describe it, to do it justice.

"I don't think there is just one reason for anybody," Brenden said. "It's just a fulfilling, emotional experience."

Most say there are some aspects of the sport that are impossible to understand unless you are the one stumbling through the night on a lonely trail, tired, hungry, fighting doubt and indescribable fatigue only to keep moving and eventually stagger across a finish line. It's about conquering your own demons, no matter what those are.

For Corbett, it was drugs.

"I spent one night in jail, and I was literally scared straight," said the California woman who has completed 250 ultra marathons, including 44 100-mile races. "Things happen for a reason, and I don't think I would have gotten into this without getting into trouble. I got a diversion and I cleaned up my life."

She became a vegan and began exercising. Friends got her into running, and it became her new drug.