CASABLANCA, Chile—There are compelling reasons for dismounting from your horse and running alongside it in the fourth of five loops in a 120-kilometer (80-mile) race through dusty hill country not far from the capital, Santiago. Especially on a mid-December day at the height of the austral summer, when heat waves are hovering above the ground. And especially when you're going downhill.

"The penultimate loop," says veteran rider and horse trainer Pedro Pablo Gomez, "is what makes Copa Chile [the Chilean Cup] a very difficult race. The horses have already run 80 kilometers, the fourth loop is the hilliest, and it's crucial we reserve energy for our last loop."

I'm in the Veramonte Vineyard, in the dramatic Casablanca Valley, the staging ground for the 2014 Copa Chile, rooting for Pedro and his horse, Otoño (Autumn). Pedro, in a long-ago race in coastal Peñuelas—which he had a crack at winning—chivalrously stopped to assist my friend who had fallen off her horse.

Endurance horse racing, with riders and horses stretched to their limits, is a young sport, like ultra-marathoning. It's growing faster than any other horse sport, with 90 races worldwide ten years ago, and 900 today.

To be eligible for the Copa, as people call it, each horse previously had to complete four endurance races totaling 240 kilometers (150 miles).

The Copa marks the end of Chile's racing season. Unlike the nine other endurance races held through the year, no points are awarded, so riders' results don't affect their international rankings.

This, says Cristian Herrera, head of Chile Enduro, one of the two organizations overseeing the race (the other being the International Equestrian Federation, or FEI), makes it "altogether more relaxed."

Casual isn't a word that comes to mind for this extreme test of the human-horse relationship. Pedro and 15 other contestants toed the starting line at 7 a.m. sharp, the condensed breaths of horses and riders mixing in the frost-laced vineyard. They wouldn't cross the finish line for at least eight demanding hours.

The course consists of two labyrinthine 30-kilometer loops through the wine estate, followed by three 20-kilometer loops, marked with colored flags. After each loop there's a mandatory half-hour stop for "rest and repair," as Pedro calls it, when a veterinarian checks the horses' vital signs and handlers water them down and let them drink their fill.

Otoño: A Chilean-Born Pure Arabian

All the horses competing today are either Anglo-Arabs or pure Arabians, the brave, elegant, unrivaled masters of distance racing.

Arabian horses evolved shoulder to shoulder with people in the Arabian Peninsula over more than 5,000 years, selected for two seemingly incompatible traits: tameness and fortitude.

They were first brought to Chile from Germany, in 1872, for breeding. An Arabian's "hot blood" has comparatively more hemoglobin than European "cold-blooded" horse breeds. Their bones are dense, but they tend to have one vertebra fewer and 17 instead of 18 ribs, making them shorter, smaller, and able to lose heat faster than other breeds. (They are, after all, desert horses.)

Otoño is a handsome eight-year-old stallion the characteristic slate color of Arabians. He was once semiwild, roaming the Talagante hills near Santiago, where he had to overcome thirst, hunger, heat, and rocky terrain.

Otoño is lucky. Spotted at a livestock fair, where he was destined for a meat factory, he was bought on the spot. Pedro acquired him soon after.

Pedro has devoted his life to Arabian horses. He's lived with Bedouin ("who have horses in their genes") in the Jordanian desert and worked for the Royal Stable of Jordan.

His stables near Santiago—called Al-Shaalan—also go by "Little Dubai," because Pedro is one of the very few South Americans who train Arabian horses for sale to sheikhs in the Middle East.

"Selling horses to Arabs is like selling ice to Eskimos," says Renni Müller, of Chile Enduro. Müller sponsored Pedro at his first world championship, in Spain, 14 years ago.

That Chile is now on par in the endurance racing world with Uruguay, Argentina, and Brazil is in no small measure thanks to Pedro. He led the way by developing a precise training regime for his horses focused on reducing injuries, and he treats Otoño and the rest of the herd like professional athletes.

The Art of Pacing

In the wild, horses naturally run together. That too characterizes endurance racing-the racers generally move in a cluster, like the peloton in a distance bicycle race.

Believing a rider must sync precisely with his horse and experience the same physical duress, Pedro cycles 180 miles a week, pedaling at a cadence that matches a horse's canter. His goal today is to keep Otoño going as much of the time as possible at a steady 18 kilometers an hour.

The "water assistant point" halfway round loop one is a good place to watch the early action. Here, two members of Pedro's support team, Juan Carlos Poblete (nicknamed "Lito"), a sprightly farrier, and Leonardo Escatarregia, once an Andean highlander running pack mules in Argentina, stand alert at the edge of the track holding up large plastic bottles filled with water.

Otoño and Pedro come into view. They epitomize composure, moving like a single entity, a superathlete centaur.

Pedro bows down gracefully on the go, grabs one bottle, then the other, and pours the cool water over Otoño's neck. They're gone in a flash, the thud of hooves receding.

We rush back to the staging area where each loop begins and ends. Horses clatter in, dust flying. Pedro dismounts, and Lito, Leo, and Jose Sarmiento (also formerly a mountain man in Argentina) splash Otoño with buckets of water. After cooling down, Otoño is led away for his compulsory checkup.

Fitness and recovery potential rather than sheer speed are keys to endurance racing: Horses' heart rates must recover to a maximum of 64 beats a minute within 20 minutes, or the horses face disqualification.

After loops one and two, Otoño's heart rate is ideal, back at 64.

The third 20-kilometer loop, marked with red flags, is the fastest and flattest of the five. "This is just biking," Pedro says coolly before setting off. But the day is heating up, testing the condition of horses and riders.

After 61 minutes, Otoño and Pedro are back at base. Leo and Lito lead Otoño off to nibble some grass in a nearby meadow, while Pedro relaxes and drinks water in the shade of his tent, lined with an Arabian rug.

"Now you'll see what endurance means," Pedro says. "Next the most difficult loop, in hills and scorching sun."

It's during this fourth loop (73 minutes) that Pedro helps Otoño by dismounting where the downhills are punishingly steep. "Trotting alongside my horse is a lot less stressful on his tendons and ligaments," Pedro says. "If more riders dismounted at this point, fewer would get eliminated." (By the end of the Copa, 10 of the 16 horses will have been disqualified.)

Finale

The wind has picked up, deriding the heat. It's after 3 p.m., and the race is now past its eighth hour. Two riders appear, neck and neck: Pedro and Andrea Fernandez, formerly with the Chilean military and known for her strict distance-running training regime.

In the final 200 meters, the two horses accelerate to a full-out gallop. Suddenly, a man laboring in the vineyard with hedge cutters—wearing headphones and unaware of the approaching horses—steps out toward them. Pedro and Otoño are forced to draw a sharp curve to avoid the trimmer by inches. That minuscule diversion puts them a head behind Fernandez and her horse, Alcazar.

Otoño finished "con alegría," Pedro says—"with joy." He added, "For that I'm proud." But I detect an edge of disappointment at his hairbreadth second-place finish.

But wait! Not 30 minutes later Alcazar is disqualified. He was assessed to be exhausted, shaking and unbalanced, run too hard. "The FEI take such a metabolic setback more seriously than lameness, as the horse can even die as an outcome," explains veterinarian German Ausset.

First place makes Pedro happy, of course. But now comes the assessment for "Best Condition."

There's a doping test. Three FEI vets check Otoño's veterinary records from the race, then poke and prod, examining everything from the mucous membranes of his eyes to the time it takes blood to refill his jugular after pressure is applied. The judges require Otoño to trot (led by Pedro) in a straight line and then in a circle on a lunging rope. He looks fresh, ears pricked, pace even, no apparent signs of weakness or weariness.

At the award ceremony, Pedro and his team collect their first-finisher trophy. Then "Best Condition" is announced. Otoño and Pedro win again.

"This is what makes sense," Pedro tells me. "It was a fair race, because Otoño won best condition. As an endurance trainer and horse lover, this is the biggest prize I can get."

Next July, in Ontario, Canada, Otoño and Pedro will race against some of the world's best at the 2015 FEI Pan American Endurance Championship.