Having run a 2:20 marathon only three weeks ago in Bermuda, Bruce Robinson, 29, of the Washington Running Club, needed an extra Inducement to win yesterday's George Washington's Birthday Marathon. His two fellow owners of a running store gave it to him: Come in first, they said, and we won't count it as a vacation day.

Running in what he called a "nice and easy pace" over the hilly and nobby three-loop course in freezing weather at the National Agricultural Research Center, Robinson finished in 2:26.22. It was his second marathon victory since he became a distance man in 1973. That year he ran this course in 3:39.

For most of the race, Robinson ran step for step with Terry Baker, a Hagerstown, Md., runner who was an all-American cross country runner at Auburn University.

In the last two miles, Robinson pulled away.

"My face was getting so cold, I just wanted to get it over with," he said in the warm cabin at the finish line where other runners, some defrosting their beards, congratulated him.

Among the women, Sue Crowe, 24, of Alexandria charged in with a 3:06.52. The biting wind, which only a herd of Devonshire hogs along the course didn't mind, gave the 100-pound Crowe trouble.

"And they tell me that this year we had good weather," she said laughing. "On the third loop, the hills become really tough in ways that aren't so bad in the first two loops."

Mike Sabino, a Baltimore physical education teacher, finished first among the 40-and-over competitors. Coming in with a 2:34.46, Sabino, who weights 118 pounds, also had trouble with the gusting wind.

"There's no question about it, it's tough to run a good time on this course," he said. "I've run here for the past 10 years, and we've had only one good day weatherwise."

The Beltsville marathon, sponsored by the District of Columbia Road Runners Club and staged by Larry Noel and Michael Horsey of the Phidippides Running Center in Rockville, is one of the oldest distance races in the country. Because of the hills and the odds-on tough weather, Beltsville has a reputation for attracting only the hearty. Of a field of 330, 225 finished.

Robinson, an affable fellow who picked up his daughter, Courtney, as soon as he dashed over the finish line, has qualified for the Olympic trials in Buffalo in May. If they are held, he may forego the Boston Marathon a month earlier.

"It would be hard to go all out in both," he said, dandling Courtney on his knee as the youngster gaped at the robust finishers who by now were piling into the cabin in large numbers.