FRONT ROYAL, VA. -- Mary Koefod should be home by now, settled back in her quality control job at Crown Cork & Seal in Waterville, Minn. She's levelheaded, so the transition should be smooth, but for a few hours earlier this week her feet couldn't find the ground. She was floating on air.

"This is my wildest dream right here," said the 33-year-old as she hugged her sweaty Arabian bay gelding, Dana's North Light. "I'm tired, but I'm exhilarated."

Why not? Minutes before, the rail-thin Koefod and her muscular mount had won the North American Endurance Ride Championship, racing 100 miles through the rocky Blue Ridge Mountains in 12 1/2 hours.

Fifty spectators waited in the gathering dusk when Koefod, Sherode Powers of California and Daralyn Butler of Texas rode into view 200 yards away. But after 100 miles and all day on the trail, who expected the steeds to gallop home like quarter-horses at a country fair?

Thunder in they did, kicking up clods of sweet-smelling mountain ground and raising goose bumps even among veterans, who expected a leader to finish alone. "Look at the hair stand up on my arms," marveled race marshal John Alexander.

It was a scene worthy of goose bumps: The great, weary, powerful beasts running big-hearted, full-bore across the meadow a length apart for a prize none of them even knew existed. It made you pause to rethink the meaning of sport, which after all is only the primeval wish to excel physically. Even a horse can feel it. "I never even hit him," said Koefod of her mount. "I just gave him a squeeze and off he went.

"I'm in love with these Arabs," she said of her horse and its breed. "It's just so impressive, what they can do."

Not to mention the riders.

Forty-two men, women and teen-agers from five nations turned up at 5 a.m. Monday to take off in the black dark and cross mountains, rivers and valleys in pursuit of the title. The only rules: Horses had to pass veterinary checks at five stops along the way and couldn't take more than 24 hours to finish. As it happened, 27 ran the route, the last rolling in at 3:10 a.m. Fifteen dropped out.

"It was the toughest trail, the nicest weather and the easiest ride I ever had," said Koefod.

So goes the sport of endurance riding, which began 32 years ago when a few lumbermen decided to find out whose mount could cross 100 miles of Sierras from Lake Tahoe, Nev., to Auburn, Calif., fastest.

The race they started -- the Tevis -- has grown since to 150 entries a year, spawned copies at home and abroad and started a national organization, the American Endurance Ride Conference. AERC sanctioned Monday's championship here, which drew riders from across the United States and Europe.

So how hard is a one-day, 100-mile mountain horseback race? "What you're looking at is people working on the cutting edge of what can and cannot be done with a horse," said Chuck Beebe, who flew in from California to work as pit crew for Maggie Price of Pennsylvania.

And how about the riders?

Consider Rocco Marabito, 38, an Italian pediatrician who came to ride a borrowed horse. When the horse broke down after 20 miles, Morabito went ahead on foot and ran another 50 miles. "I just wanted to see the course," he said.

"Let me put it this way," said race official Marie Dey, a woman in her 50s, "I was doing a 50-miler in New Jersey and my horse threw me. I broke three ribs, cracked an ankle and had a concussion. When I pulled into the next {veterinary} stop, I asked the vet, 'Is my horse okay?'

"He said, 'Yeah, but you're not.' I said, 'Just strap me on.' They did, we finished, and that horse was reserve champion," finishing second.

Stories like these are legion among endurance racers. But while they don't mind beating themselves up, the riders say they'd sooner die than hurt their horses.

To that end, races like the North American and the Tevis are overseen by veterinarians who stop the action to inspect horses at prearranged spots. The rules are inviolable: Pulse must recover to 64 beats per minute from a running rate of over 100 and the steed must pass a full veterinary check. Then horse and rider rest a mandatory 10 to 30 minutes before carrying on.

When they finish, horses again must recover to a 64 pulse within a half-hour and, in the North American, they were checked again the following morning. Two showed up lame and were eliminated.

It makes the event as much a test of conditioning as talent, and it can take two years or more to prepare a horse for a 100-miler. Are these rules excessive?

"The horses are defenseless," said Matthew Mackay-Smith, a veterinarian and Hall of Fame endurance rider. "The riders love them, but they still need all the attention they can get."

To wit: This summer a nonsanctioned endurance race in Oklahoma ran without veterinary checks and several horses were ridden to their deaths, said race organizer Beverly Fields. "That's jackpot rides, the way it was in the Wild West," said Mackay-Smith. "Ranchers thought of horses as equipment and if they broke down it was the same as a haymow breaking."

Unlike the cowboys of the Wild West, today's endurance riders in sanctioned events are mostly professionals from both sexes and all ages (from 16 to 69 in the North American) who consider their horses the star athletes, not themselves. The riders wear astonishing get-ups -- running shoes, Spandex tights, pantyhose, sweatsuits, helmets, bandanas, anything that works. Many are refugees from the high-powered world of show horses.

"Showing is backbiting and political. There's too much money involved," said ex-show rider Price. "This is sportsmanship. Everyone here wants to win, but if you're stopped by the side of a trail, not a rider goes by that won't say, 'Anything I can do to help?' "

"It's a wonderful sport," said Koefod. "You don't need a lot of money, you don't need fancy equipment and you don't need a registered horse."

All you really need, said the riders, is heart. And a horse with heart.

And plenty of it.

The host club for the North American was the Old Dominion 100-Mile Endurance Ride, Inc., which, if all goes well, will stage the world endurance ride championships here next summer. That's why riders from Europe made the trek over to scout the course. The annual Old Dominion 100 is held in June.

For information on future races, write Old Dominion 100, Rt. 4, Box 140, Front Royal, Va., 22630.