At 9 o'clock this morning, the simultaneous sounding of a starter's pistol and a downrange 105-mm howitzer at the Iwo Jima Monument in Arlington will launch a pack of some 7,000 runners, and maybe more, on a 26-mile, 385-yard race trough streets and parks of Washington and Northern Virginia.

Officials of the third annual Marine Corps Marathon had processed 6,283 entries, including 453 from women, by Thursday afternoon. They decided to keep accepting late entires until an hour before the starter's heavy artillery fired this morning. Vehicle traffic in front of the monument will be closed at 8:15 a.m.

"We think we might go over 7,000 runners. That would make us one of the three or four biggest marathons in the country, as far as participation," said Capt. Jim Burke, the race coordinator.

Kevin McDonald of Greenville, S.C., who won the stirring "run through the monuments" last year in 2 hours 19 minutes 36 seconds, is not defending his title. However, several 2:20 marathoners head the massive field.

The best career time (2:18) belongs to Phil Camp, 31, a Navy helicopter pilot from San Diego who is stationed in Milton, Fla. He finished second last year, tweaking marines by finishing first among military entries in a "Fly Navy" T-shirt.

Charlie McGuire, 26, of State College, Pa., an academic advisor at Penn State University, has the best time of the civilian runners who make up two-thirds of the field. He ran 2:18:22 in a marathon at Phoenix in 1975. Kevin McCarey of Villanova, Pa., is the only other sub-2:20 man in the field.

The local favorite is Max White, 27, of Alexandria, a math teacher at the Episcopal High School. He finished third last year in 2:21:32. His best time was 2:20:40 in the 1975 Boston Marathon. He prefers to ignore what the opposition does and run a steady 5:15 mile pace from start to finish.

Dan Rincon, 25, of College Park, was an All-America in cross-country at the University of Maryland. He has stepped up his distances since being graduated in 1975 and ran 2:20 in the 1977 Boston Marathon.

Other 2:20 marathoners entered are Chuck Burrows, 30, McGuire Air Force Base, New Jersey; Robert Clerk, 41, Phoenixville, Pa.; and Scott Eden, Durham, N.C., a medical student at Duke.

Jane Killion, 29, of New York, third among women in the Boston Marathon this year in 2:47:23, has the fastest time of the female entrants.