WASHINGTON -- Thousands of people are expected to take part in the Wirefly National Marathon Saturday morning in the District.

The marathon starts at 7 a.m., and the 26.2-mile race begins and ends at Robert F. Kennedy Stadium.

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About 4,500 runners are expected to participate. About 2,800 runners, including D.C. Mayor Adrian Fenty will take part in the half-marathon. The others will run in the full marathon.

There will be a lot of road closures associated with the race as runners make their way from the stadium through Capitol Hill and through parts of Northwest, Northeast and Southeast.

The most difficult part of the marathon comes at the Calvert Climb. It's a tough hill leaving Rock Creek Park between miles 19 and 20 of the race. In a quarter mile, runners have to climb more than 100 feet.

Some of the major roads that will be closed are Pennsylvania, Constitution and Independence avenues, as well as parts of the Rock Creek Parkway.

Metro officials will reroute 37 bus routes because of the race. People are being encouraged to use Metrorail, which will not be affected. The city paid Metro to open an hour early at 6 a.m. to accommodate runners and spectators.

Runners can get to RFK Stadium by way of the Stadium/Armory Station. For those runners who have to drive, organizers said there will be free parking in lots 6, 7 and 8.