Speed. It is the quality most prized in a thoroughbred. Not only does it win races, but possession of raw speed usually suggests that a good racehorse will become a successful sire.

Yet trainers look warily on horses endowed with blazing speed. They know that speed, like nuclear energy, is powerful when it is harnessed and employed properly, but potentially destructive when it is not. The pages of racing history are filled with the names of brilliantly fast horses whose speed could not be controlled, and who were nothing more than sprinters as a result.

The subject of speed is on the minds of most racing fans at Gulfstream Park, as we await the most intriguing horse race of 1982 to date. In the Fountain of Youth Stakes on Monday, two extraordinarily fast 3-year-olds, Distinctive Pro and Star Gallant, will be attemping to run 1-1/16 miles for the first time. Both horses have performed so well in sprints that they will be prime contenders in the Triple Crown races if they can sustain their speed over a route. Star Gallant, in fact, has the potential to be an all-time great racehorse--if he can go the distance.

What determines whether a fast horse can run a classic distance? Most racing fans would not hesitate to answer: breeding. In England, in fact, students of the game are so attuned to the distance capabilities of different bloodlines that they might watch a horse win magnificently at 1 1/4 miles and proclaim that his pedigree would make it impossible for him to go 1 1/2 miles.

American race-trackers aren't quite as rigid about pedigrees, but they know that there are plenty of bloodlines in this country that produce only sprinters. The best 3-year-old in Maryland, Shimatoree, may be as fast as Distinctive Pro and Star Gallant, but it is hard to take him seriously as a Triple Crown candidate. His sire, Marshua's Dancer, never won a race longer than seven-eighths of a mile, and his progeny are rarely effective beyond that distance. Shimatoree will probably prove to be a victim of his genes.

Yet very few horsemen and women, if asked why certain speedsters can go a distance and others can't, would cite bloodlines as the determining factor. They see much more of a correlation between the horse's personality and his route-running capabilities.

"The ability to relax is what makes a speed horse able to go a distance," said Walter Blum, the former jockey who was renowned for his success with front-runners. "It's temperament more than pedigree that determines whether they can.

"I rode Distinctive Pro's grandsire, Distinctive," Blum recalled, "and he had a great burst of speed, but he'd get the message to relax and he'd settle into a smooth stride. I rode his father, Mr. Prospector, too; he was quick and high-strung, and you couldn't rate him. When you wanted to go slow, he'd want to fight you. Distinctive Pro acts more like his grandsire; he has great acceleration from a standing start, but once he's in stride he rates real kindly."

Because of these differences in temperament within the same family, Distinctive Pro was able to win stakes at 1 1/8 miles, but the more gifted Mr. Prospector could go much beyond six furlongs. Of course, nobody knows for sure about Distinctive Pro yet.

Most trainers agree that the ability to relax is crucial when a speed horse is trying to negotiate a distance, but they don't view this trait as something that is necessarily inborn.

"The most important factor," said Calumet Farm's John Veitch, "is the man (or woman) training the horse. Some have got the knack for getting a horse to relax so he can carry his speed over a distance."

There are as many ideas on how to accomplish this as there are trainers. "I believe in giving the horse long gallops in the morning and not concentrating at all on his speed," said Veitch. "You want to develop their breathing so it will sustain them at a distance."

Woody Stephens, the Hall-of-Fame trainer, suggested, "When you work the horse, you try to ease away from the pole rather than fly away and go the first furlong in 11 seconds. The exercise boy can be very important in teaching a horse to relax. The groom can be important, too: an easygoing type may help keep the horse from being excited."

Jimmy Jones, who trained for Calumet Farm during its heyday, maintained, "Most horses who can go seven furlongs or a mile can go a mile and an eighth, a mile and a quarter, or more if they're developed properly. But they have to be taught to conserve their energy, and most people are not teaching that anymore. What we did was to come here with the 2-year-olds in the winter with no idea of running them. We spent time breaking from the gate, letting them go head and head. That had much to do with their going a mile and a quarter later on."

A trainer's efforts to alter a horse's instincts and control his speed do contain elements of risk. Two summers ago, the most promising 2-year-old in America appeared to be a colt named West on Broad. Before he went to the post as the favorite for the prestigious Hopeful Stakes at Saratoga, trainer Lou Rondinello looked ahead to the future and said, "We're going to have to get the speed out of him."

Rondinello did this, but in the process he also got most of the stamina and other signs of talent out of West on Broad. The colt never developed into anything more than a run-of-the-mill allowance-class sprinter.

It is no coincidence, then, that almost every speed horse who has developed into a classic-winning champion in recent years has been managed by a first-rate horseman.

Probably the archetypal speed horse of the modern era was Seattle Slew. He was fast enough to be a champion sprinter, but he was just as effective at a mile and one half. His only drawback was his extremely competitive nature. In the manner of other great front-runners such as Never Bend and Dr. Fager, he sometimes dissipated some of his energy fighting off challenges in the early stages of a race.

Affirmed was the only other speed horse to be voted Horse of the Year in the last decade, but he was much different from Slew. Affirmed was quick enough to win sprints wire-to-wire, but he was perfectly tractable; a jockey could place him anywhere, and Affirmed would cooperate. He was a trainer's dream-and an extreme rarity.

It is conceivable that either of the principals in the Fountain of Youth could be another Seattle Slew or Affirmed. But if they are not, their trainers can hope to make them into another Bold Forbes. An incredibly fast sprinter, Bold Forbes seemed so uncontrollable that it appeared that he would never be anything more than a sprinter. But his great trainer, Laz Barrera, worked diligently to teach the colt to relax and "to get the crazy speed out of him."

He succeeded, and even though Bold Forbes was not truly cut out to be a classic horse, he won the Kentucky Derby and the Belmont Stakes. Barrera's feat demonstrated that when a horse is endowed with such great speed, almost anything is possible.