In Michael Horsey's 35 years at Georgetown Prep, senior Steven Duplinsky is the first student he has seen who can study for two hours straight while training on a stationary bicycle and his heart working at 130 beats per minute.

"I've never seen anyone as focused as Steve," said Horsey, who is admissions director at the Rockville private school and Duplinsky's adviser.

With that determination, as well as tremendous endurance and talent, Duplinsky has become a world champion triathlete and, with four dazzling performances this fall, the most dominating cross-country runner in the Washington area.

On Sept. 10, Duplinsky won the junior division of the World Triathlon Championships in Gamagori, Japan, by 25 seconds against a field mostly comprising professionals. A week later, without having run at all since that triathlon and still sore from the long flight home, Duplinsky ran one of the fastest times ever (16 minutes 3 seconds) on Hereford High School's notorious three-mile course in Parkton, Md., to beat a pair of distinguished All-Mets, Chantilly senior Brad Siragusa and Broadneck junior Matthew Centrowitz, by more than 40 seconds.

In October, Duplinsky broke the course record by 14 seconds in winning the William & Mary Invite; he won the Eastern States championship at Van Cortlandt Park in New York by seven seconds against elite East Coast competition; and last Saturday at Landon he won the Interstate Athletic Conference title by a minute.

"It has been a joy to watch," Georgetown Prep Coach Richard Drozd said. "It's not just the times he's running, it's the times in the context of his competition. He is beating some very good runners by a lot."

Since running his first triathlon in middle school, Duplinsky's ascent has been rapid. He won the 15-and-under national championship in the triathlon three years ago, and he was an All-Met in cross-country the last two seasons. He has barreled through the biggest obstacles in his career, including a bout of mononucleosis two years ago and a stress fracture that cut short his outdoor track season last spring.

This summer, Duplinsky gave his full devotion to training for triathlons -- a 7,500-meter swim, 20 kilometers on the bike and a five-kilometer run -- spending most of his time in the pool or on the stationary bike.

Duplinsky estimates that he spent 24 to 28 hours a week in training for almost the whole summer, time spent mostly by himself. That singular determination paid off in Japan, where Duplinsky won the triathlon by 25 seconds, capped by a 15:10 five-kilometer run.

"I don't think about it as much as I did right after it happened," Duplinsky said. "But every once in awhile I'll think about it again, and I still get chills."

Perhaps the most impressive aspect of Duplinsky's cross-country success this year is that he has done it despite relatively little training. Only in the last few weeks has Duplinsky focused fully on running, with the hope of succeeding at the Foot Locker Nationals. The Northeast Regional race is Nov. 26 at Van Cortlandt Park, and the top 10 finishers qualify for the national finals Dec. 10 in San Diego.

"I'm just not in great cross-country shape yet," Duplinsky said in the week following Eastern States. "My strength isn't there yet."

Yet because of his incredible fitness from the training and nearly flawless running mechanics, Duplinsky is still many strides ahead of the rest of the competition. As he did for most of the summer, Duplinsky trains on his own for cross-country.

"I actually enjoy" training alone, Duplinsky said. "It's how I've been doing it the last couple of years. I get to see how far I can push myself."

With an unassuming, humble demeanor and a good sense of humor to go along with a graceful running style, Duplinsky has earned the admiration of his peers and other coaches. At the IAC meet, Duplinsky crossed the finish line in 15:08 to resounding applause from the crowd.

"He's just is so relaxed out there," Landon Coach Addison Hunt said. "It's the same way the best athletes in the world look when they're running, like it comes naturally. It's a beautiful thing to watch. . . . When you're watching him, you get the sense that you're watching something special, that you'll be able to tell people later, 'I saw him when he ran in high school.' "

Steven Duplinsky, running away to victory at the IAC cross-country championship last weekend at Landon, has also become an accomplished triathlete.