As the weather cools and the New York and Marine Corp marathons approach, the road racing season is in full bloom in Washington, with the most numerous and diversified showcase of races all year being run today.

For the ultramarathoner, the 10th annual National Capital Challenge 36-Mile Run is the perfect distance. Beginning at 8 a.m. at the corner of North Union and King streets in Alexandria, the race proceeds to Mount Vernon and back, then winds through the District before taking the bicycle path back to the Alexandria for the finish.

More than 200 applications have been received for the race, which is patterned after the Two Bridges 36-Miler in Scotland. Race-day registration is available. Charlie Trayer of Reading, Pa., the 1981 and 1983 winner, will not defend his championship, choosing to rest after a third-place effort in Scotland last August.

Last year's runner-up, David Horton of Lynchburg, Va., could break Trayer's course record of 3 hours 31 minutes 34 seconds set in 1981. Patricia Howard of Burke and Stephanie Shipp of Great Falls are the women's favorites.

An addition to the race this year is a 20-mile distance.

For more than 200 women, the Hecht's/9 West Women's 4 Mile Run starts at 8 a.m. at Seventh and Water streets SW. Two-time track and field Olympian Lacey O'Neal is spokesperson for the second annual event, to benefit D.C. Special Olympics and D.C. Department of Recreation/Therapeutic programs.

The race is part of a six-city series of women's four-milers sponsored by 9 West. In the first running, George Mason University's woman's cross-country team swept the top places, but it will not be returning. The winner receives a weekend trip for two to San Francisco and the runner-up earns a four-day trip for two to Jamaica. Registration will be available today.

Other area races include the fourth annual Fritzbe's 10-Kilometer run in Reston and the Mobility 10K road race at Hains Point, both starting at 8 a.m. The fourth annual Fritzbe's event, attracting more than 1,400 runners, includes a race and a brunch, but no race-day registration.

Runners can register today for the fifth annual Mobility run, with the proceeds of the more than 300 participants benefiting "brittle bone disease" research at Children's Hospital.

Also, the fourth annual Fairfax Symphony 10K will begin at 9 a.m. at Tysons Corner. Runners may register at the race.