The gun, in this case a 105-mm howitzer, blasts at 9 a.m. today to start the fourth annual Marine Corps Marathon.

Nearly 8,500 runners including almost 6,000 civilians, and 1,046 Marines, are expected to run the 26-mile, 385-yard course that is billed as the "run through the monuments."

Scot Eden, 26, a medical student at Duke University, who set a course record last year (2:18:08), is the first returning defending champion. Eden set a personal record, 2:16:21, in January at the AAU Nationals in Houston.

Eden, who is taking a lighter load in school this year in hopes of making the olympic team (he admits he is a "very long shot") expects go out at a 2:16 pace.

Max White, 28, the local favorite from Alexandria, says he expects Eden to win. But White, who won the Dannon 36-mile Ultramarathon on Oct. 14, and tied for first in a 15-kilometer race the week after (48:28) says he feels "confident."

White says that a lot of runners, including himself, "will be interested in Olympic trials" next summer in Buffalo (the required time is 2:21:54). White's best time in the marathon is 2:20:40.

He believes this "is the strongest field the Marines have ever had," it includes Phil Camp, a Navy pilot from San Diego, who placed second in 1977 and has run a 2:18; Peter Nye, of Alexandria, whose best in a 2:23, but has run very fast at shorter distances; and Will Albers, of Fairfax, who, White says, "is not a marathoner but has been tearing up 10-mile races."

The field also will include a team from the British Royal Air Force. One member of the team, Michael Hurd, has run a 2:15:54, the best in the field.

Among the starters will be 55 runners whose best previous times are under 2:30 and 10 under 2:20. Four of the women who have entered have run under 3 hours. But defending champion Jane Killion (3:02:34), of New York City, will not run.

The following streets will be closed, blocked or congested this morning because of the marathon. In the District, two eastbound lanes on the Key Bridge and Whitehurst Freeway closed 9:15-10:45; D and 1st streets NW to D and 1st streets NW to D and 1st NE. closed 10 a.m. noon; Ohio Drive, from South Tidal Basin to Buckeye Drive, blocked 10:15-3 p.m.; north parkway from 395 south blocked 10:45-3 p.m.; the south parkway ramp from 395, south blocked 10:45-3 p.m.: 395 from the south parkway blocked 10:15-3 p.m.; traffic along District cross streets from Rock Creek Parkway to the Capitol restricted 9:45-2 p.m.

In Arlington, Marshall Drive closed 7:00-3 p.m.; Virginia Rte. 110 closed 8 a.m.-3 p.m., motorists may use George Washington Parkway for North South access.