In four months, Margaret Groos has converted herself from a good 10,000-meter runner to America's top women's marathoner.

Groos earned that distinction Sunday when she ran the fastest American time in more than two years while winning the the U.S. Olympic Women's Marathon Trials in 2 hours, 29 minutes, 50 seconds.Competing in only her third marathon, Groos, 28, broke the Pittsburgh Marathon course record of 2:31:53. Her time also was the first sub-2:30 run by an American woman since 1984 Olympic champion Joan Benoit Samuelson clocked her U.S. record of 2:21:21 in the 1985 Chicago Marathon.

Groos owes her effort to the hard work she has put in since relocating to Tallahassee, Fla., from Nashville, Tenn., in January.

"I haven't missed a hard workout in the last three months," said Groos, who last ran a marathon at the '84 Trials but qualified for the race by posting a sub-33:00 at 10,000 meters. "I haven't missed one day. I've been concentrating on long runs. I worked on starting steady and ending strong."

Her dedication has rewarded her with a trip to September's Seoul Olympics. Joining Groos on the Olympic team will be Nancy Ditz of Woodside, Calif., and Cathy O'Brien of Boston, the second and third finishers.

Samuelson, who pulled out of the race last week because of a hip injury, may appeal the U.S. Olympic Committee for a special spot on the team.

Groos overcame a thyroid problem after finishing fifth at the '84 Trials and she has struggled to build herself up. Her weight fluctuated until she could regulate the disorder.

"It's been tough battling it," said Groos, who has shed almost 10 pounds since beginning training in January. "This is a big mental breakthrough. Now I have the confidence to go back and run a lot faster."