Dan Rincon listened carefully to the weather report Saturday night. The forecast: 90 degrees, and a very tough race.

But Rincon set a blistering pace for a blistering day in winning the third annual Hecht Company 10-miler yesterday in Rock Creek Park. His time was 50 minutes flat. His personal best in 47:49.

"When I heard the forecast," he said, "i knew anything close to 50 would be pretty good."

Rincon, 27, of Wheaton, took the lead about two miles into the race. "Bernie Allen was the only challenger but he dropped off fast," Rincon said. ""I stepped on the gas, and tried to get some daylight because later I knew it would be tough to do."

Rincon, who runs for the Washington Runners Club, had close to a two-minute lead as he approached the steep hill leaving the park around the right mile mark. "My splits were sub-50 (sub-five-minute miles) before then," he said, "but the hill really put it to you."

Peter Squires of Yonkers, N.Y., who won the race in 1978, finished second in 51:54. Brent Hawkins, of Uniontown, Pa., was third in 51:57.

Mary Walsh, who finished first among the women in 59:07, did not realize she was first. "I was just looking to finish," she said.

Walsh, 19, who runs for the University of Maryland, said the weather did not bother her. "Beautiful day," she said. She wasn't kidding. She then excused heself to go jogging."

Kim Dunlap, of Gore, Va., was second in 60:35; Mary Ellen Williams, of Rockville, was third, in 62:31.

A total of 2,400 runners started the race and an estimated 2,000 finished.