IT'S TOO EARLY TO TELL how many converts Karnazes and Reed will bring to ultrarunning, since it takes newbies a solid year to train for their first race. But no one can deny that Karnazes is revolutionizing the sport, inspiring many weekend warriors to take it up a notch.

"We'll see bigger numbers," says Western States race director Greg Soderlund, "and a lot of that is because of Dean's book." Reed, too, wants to spread fitness fever. "I want to be an inspiration to girls and other women that they can do anything they want," she says, "and not just let categories like 'mother' limit them."

Money and fame aside, both Karnazes and Reed are motivated more by primal need than anything else. For years they toiled in obscurity, running simply because they couldn't stop. On the surface Karno is a jokester who says he has no problem balancing his training schedule, his job, and his family life. But an easygoing ultrarunner is an oxymoron: On a typical weekend, he'll run all night and play normal dad during the day. And easygoing people don't generally quote Dostoyevsky ("Suffering is the sole origin of consciousness") or write things in their book like "If you're not pushing yourself beyond the comfort zone . . . you're choosing a numb existence."

Reed races almost monthly, because she is, by all appearances, completely obsessed. She simply can't sit still: If she's at an airport, she runs laps around the parking lot; if she's in the car with her family, she gets out while they eat, and they pick her up down the road an hour later. "I might have ADD," she says. "I'm very hyper. I've never not run, even two days after my C-sections."

It's possible to view Reed's drive to outrun Karnazes as a spin cycle of desperation. No matter how many races she wins, she'll probably never achieve his media stature, and she'll put her body through hell in the process. In other sports, ability is determined by points or scores or time limits, but in ultrarunning, rogue competitors like these two could theoretically just keep running until they drop.

The jury is still out on how running ultras may affect bodies like Karnazes's and Reed's. Over the short term, it's not uncommon for runners to seize up, retch, or pass out. Some suffer acute renal failure caused by tubular necrosis (when waste from muscle breakdown infiltrates the kidneys), or hyponatremia (a life-threatening low sodium concentration). On a 100-mile run, racers can lose and consume up to one-third of their weight in fluids and damage muscle tissue to the extent that it can take months to recover. They may suffer "leaky gut" syndrome, in which intestinal contents make their way into the bloodstream, resulting in nasty digestive problems. And they may be suppressing their immune systems, aging prematurely, and lowering their sex drive.

As a woman trying to prove herself against men, Reed pushes it even harder. Although she works from her home office as the race director of the Tucson Marathon, she still runs carpools, gets dinner on the table, and attends Jackson's Little League games. Further complicating the picture is her history of anorexia. Off and on for 15 years, starting in 1976, Reed struggled with the disease, taking laxatives, except while pregnant. In 1991, she quit after it finally dawned on her, during a hospital stay, how skeletonized the bodies of other anorexics were.

"It was so stupid," she says. "Just stupid. I was like, Never again." So she replaced one compulsion with another, relatively healthier one. And she's taking it as far as she can.