At 15, Mark Johnston could hide his burning ambition to become a jockey no easier than he could hide his feet. A look at those size 7s provoked advice that he give up trying to ride, that one way or another he was bound to be too big to be a jockey.

"I was 5 feet and had these big skis," he said. "Everybody said: 'This kid'll never make it. He's going to be a giant.' "

Four years later, he is towering above jockeys in other ways, having ridden more winners this year -- 249 -- than anyone else in the land.

Maryland has become known as a showplace for young riders, producing three of the last four Eclipse Award-winning apprentices. Johnston, the early favorite for this year's title, has given the display new dimension.

"I think I was 5-8 the last I checked," he said, which probably was a good inch ago. Since he came from Kentucky in November, Johnston has consistently under-reported his height, as if that would make him more acceptable in his field.

His size may be unusual for a jockey, but his professional growth is remarkable, considering his first ride at a major racetrack came less than 15 months ago, a $49.80 winner at Churchill Downs.

"I was along for the ride," he said. "From working horses in the morning, I knew what I was doing, but as far as race-riding I had no clue. I was just out there. . . . That stretch seemed like the longest I ever rode in my life. I'm on the front end and I keep looking back. 'Where are these guys? Did I drop 'em leaving there, or something?'

"At the sixteenth pole, all I could hear was sticks, but I never did see 'em. Pulling up, I still can't believe it. I come back, the first thing I ask is if they put up an inquiry. That's probably my greatest win."

This was how he had yearned to make a buck since the day his mother took him to the races, at Keeneland in Lexington, Ky., when he was 10. Six years later, the track became his teacher, making school expendable after 10th grade.

If Johnston did not have a master plan in his earliest days of race-riding, he quickly adopted one. Now, from turn to stretch, he sits poised beyond his years. Fledgling jockeys often are slaves to impulse, moving too early or too aggressively. Even when he first arrived at Laurel -- with less than six months of experience -- Johnston seldom overreacted.'He Sits So Nice'

"He has good patience and soft hands. That's why I use him," said John Hartsell, one of Maryland's top percentage trainers. "He's a tall, lanky kid, but he sits so nice and so still. Horses run for him."

Johnston, 19, may not be low in the saddle, but neither is he upright. He has no trouble bending his spindly legs enough so his back is parallel to the ground.

"I really have to duck my head down more," he said. "As long as you've got your head down, the air just sort of flows around you. It keeps you aerodynamic. The worst way you can sit on a horse is if you're like you're sitting in a chair. That's when you catch all that wind. My legs are a little longer, so I might be just a little higher in the saddle than most riders, but also horses relax good for me like that. I think if you can get a horse to relax for you, that's half the battle."

After years of informal training atop quarter horses on his uncle's Lebanon, Ky., farm, Johnston honed his craft at Keeneland and Churchill Downs, where he observed the king of cool, Pat Day. However, he says he made no conscious attempt to copy Day (the nation's second-ranked jockey by wins), that the patient way evolved by instinct.

"I think it's got to be natural," he said. "A lot of 'bug' riders, they're real impatient. They get in a spot, and instead of sitting and waiting for something to happen, they try to make something happen. You get yourself in trouble doing that."

Johnston's riding style seems an extension of character. He has a sleepy way about him. His blue eyes blink slowly. He has a long, ambling stride. His voice is stolid, with splashes of down-home flavor. His self-belief is there, although at times it's tough to find beneath so much humility.

"The way he seems is the way he is," said Allen Stacy, Johnston's friend and housemate. "He's low-key, nice and calm, well-mannered, a real pleasant kid."

"On the racetrack, I'm patient and all that, but when it comes down to it I'll do anything to get there as long as it's legal," Johnston said. "I know where that fine line is. I've been lucky enough and smart enough not to cross it."

In more than a thousand mounts since he came to Maryland, Johnston has yet to be suspended for riding recklessly. However, he was fined $1,000 last week for being too coy. Thinking One Third would not be caught in a $19,000 allowance race at Laurel, he pulled back the reins in midstretch, expecting the filly to coast to victory, then watched in horror as McKilts came rushing outside of her to win by half a head.

The rookie mistake left Johnston saying: "It's an expensive lesson, but what really hurts is knowing I let so many people down." Much was risked on One Third, the 11-10 favorite that day.

Detractors say Johnston is winning so frequently because he is the No. 1 rider for the circuit's most proficient trainer, King Leatherbury, and therefore can afford the finesse. Stacy, the champion apprentice of 1986, said: "He's riding A-1 stock now, but when he loses the bug {apprenticeship} there'll be a lot more 15-1 shots, like the ones I ride.

"He'll learn to be a better rider after the bug because of that. He's learned how to keep the best horse from getting beat, which is a lot easier than getting an inferior horse to win. He'll probably drop off a bit, but he won't go dead. I'll be real surprised if he goes dead."

Five pounds might not seem significant, but to many a jockey it has meant the difference between strut and strain. Apprentice riders get a five-pound reduction to offset their lack of experience. Take away that advantage, and they have a way of becoming invisible. 'I've Learned a Lot'

Johnston believes his fiery baptism will help forge his future. "I've learned a lot because they ride me hard. They don't ride me like just any other bug rider," he said. "They ride me like they would ride {Kent} Desormeaux. Sometimes they ride their races according to me, knowing that I'm sitting on the 6-to-5 shot, and they kind of watch for me -- try to stop me on the fence or maybe drift me out a little bit. But I think that's been good for me. It'll help me as a journeyman."

On Thursday, the day Pimlico opens its summer meet, Johnston will lose his best friend, "the bug." Two factors should aid the transition to journeyman: His savvy horsemanship and Leatherbury's endorsement.

Riding first-call for Leatherbury is the most coveted assignment among Maryland jockeys. Leatherbury might not have an arsenal of stakes horses, but day to day he wins more races than anyone else. That not only assures a jockey quality mounts, it promotes wealth and visibility.

According to Leatherbury, the job is Johnston's to hold or fold once his apprenticeship expires.

"He has the style of riding that I like," said Leatherbury, who has employed such accomplished apprentices as Desormeaux and Chris McCarron. "He's under control out there, thinks things out, holds a horse together real well, doesn't use 'em up, sits cool. I'm going to give him an equal shot. The horses he's been riding and riding well, he'll stay on."

On the upside, losing the bug will allow Johnston to pack a few extra pounds. At about 5-9, he weighs 106 in order to tack 109, and has managed it without great reliance on the steam room. He over-ate July 11, however, spent the next morning in the hotbox (before riding three winners), then went home sick before the races July 13; the track's first-aid station reported him weak and dehydrated.

"When {Johnston} first started riding here, it wasn't so much his height that most {riders} were talking about," said jockey Donnie Miller Jr. "A lot of them were questioning how long he'd be skinny enough to do the weight."

Since his earliest inclinations toward the sport, Johnston has had to justify his qualifications, although certainly not to himself.

"I know when I got here, people were saying how I was too tall to be a good rider," he said. "It was just another situation where I had to prove myself. Now I'm losing the bug, and people are going to start talking again. That doesn't bother me. If anything, it gets me motivated. Just keep telling me I can't."