Marathon fever, the most interesting symptom of the national running epidemic, infected New York two weeks ago. Tomorrow, a somewhat milder but still noteworthy case will hit Washington.

Nearly 7,000 runners, one of the largest fields for a footrace anywhere, have entered the third annual Marine Corps Marathon, which starts at 9 a.m. tomorrow at the U.S. War Memorial (Iwo Jima Monument) in Arlington.

The scenic 26-mile, 385-yard course winds through streets, over bridges and across parks in the District and Northern Virginia before finishing back at the striking monument that Marines call simply "the Iwo."

The Marine Corps Marathon has not yet gripped the nation's capital as completely as the New York City Marathon - which attracted 9,875 starters, including many of the world's leading distance runners, and nearly 2 million spectators - has captivated the Big Apple.

It seemed as if everyone in New York a fortnight ago knew and cared about the race through the city's five boroughs. It was a civic happening, made possible only by the rare cooperation of approximately 50 municipal and private agencies.

The crowds that lined First Avenue in Manhattan and the finishing West Drive in Central Park were more than could be accommodated in any stadium in the world.

Formidable throngs lined most of the route through diverse and colorful neighborhoods. Many onlookers carried signs of encouragement, some of them for the mass of runners, others personalized. A young man outside the Rainbow Tavern on Fourth Avenue in Brooklyn, for instance, held slott a touching message: "May the Force be with you, Dad."

Washington has not yet embraced the Marine Corps Marathon so fervently, but give it time. New York's first marathon, run entirely in Central Park in 1970, attracted only 126 runners and handsful of curious observors.

The first Marine Marathon, a rather haphazardly organized affair, had 1,600 entires. Last year, the field was 3,633. This year, it was grown exponentially

About one third of the entrants are military personnel, including 1,100 Marines. Civilian entries cut across economic and social boundaries, including 470 lawyers, 465 college students, seven cooks, three painters, 21 secretary, 387 teachers, 15 nurses and one mortician. A Marine Corps census of the entry list notes that there are no lighthouse-keepers running.)

Organizers are expecting upwards of 30,000 spectators along the route. The biggest crowds are expected to gather around the start/finish point, but other prime vantage points include Key Bridge (nine miles, leaders arriving about 9:45 a.m.), Lincoln Memorial (11 miles, 10 a.m.), the U.S. Capitol (13 miles, 10:15), and Hains Point (20 miles, 10:50).

The Marine Marathon does not have a world-class field like New York's because the Department of Defense Code of Standards precludes commercial sponsorship or payment of expense money to runners. The New York race has $300,000 in sponsorship money, and spends much of it flying in the best distance runners from around the world.

Kevin McDonald of Greenville, S.C., who won last year's race with a time of 2 hours 19 minutes 37 seconds, decided not to return when informed his expenses could not be subsidized.

The timing of the Washington race - coming just two weeks after one of the three premier annual marathons in the world: the other two are Boston and Fukucka, Japan - precludes the entry of all those leading runners who go to New York, including such local standouts as Bruce Robinson. Wise runners do not attempt two marathons in two weeks.

"It takes two or three weeks to recover fully." There's a lot of damage done to the muscles. Running 26 miles at a five-minute-mile pace tears the body down, and it takes time to recuperate," says Max White, 27, the math teacher from Alexandria who is among the favorites to win here. He has been training six months for this race, running 120 miles a week, and experts to do steady 5:15 miles from start to finish.

Despite the drawbacks, the Marine Marathon has a great deal going for it, including a spectacular course, eager organizers, and the special spirit provided by an intense inter-service military men entered.

The course - flat, fast and inspiring - is conducive to personal-best times. Seven of the men entered have run marathons in better than 2:21 Phil Camp, 31, a Navy helicopter pilot from San Diego who finished second last year, has the career best of 2:18). The winner can reasonably be expected to come in under 2:20, especially if the weather is cool.

The race starts on Route 110 (the road will be closed at 8:15 a.m., and all six lanes used) near the Iwo Jima Memorial. It goes around the Pentagon and back along Arlington Avenue, over Key Bridge and through Georgetown, past the Kennedy Center and Lincoln Memorial, along the Mall at Consitution Avenue to Capitol Hill, around the Capitol and Union Station, back along Independence Avenue to the Tidal Basin, around East Potomac Park and over the George Mason Bridge, and along the Potomac River back to "the Iwo."

Boston's celebrated marathon, which now draws a million spectators each Patriots Day, has its "Heart-break Hill." New York his its cobblestone streets and distinctive ethnic neighborhoods. But the Washington Course is as picturesque as any. It was carefully plotted to accentuate the best of the city's rich architecture, and has quickly gained a national reputation as "the run through the mosuments."

If the stirring scenery isn't enough to keep wearying runners psyched up, the Marine Drum and Bugle Corps. and five high school bands will provide rousing music at the start, Key Bridge, Lincola Memorial, Washington Monument, the Capitol and the finish.

A couple of changes in the course should alleviate the aggravating traffic snarls that occured last year, and prevent what happened to luckless Marine Capt. Juan Garza. He was struck by a car at 15th Street and Independence Avenue, knocked to the pavement and out of the race. (Garza, who was near the lead for 13 miles until he ducked into Union Station to use the men's room and later had his mishap, is not entered this year).

"The 9th and 12th street under-passes will remain open to traffic this year, the race route avoids them, and that should faciliate things," said Capt. Jim Burke, the marathon coordinator. "The flow of the marathon is east-west, and the problem last year was that if pretty much stopped north-south traffic in the city for a couple of hours."

Notable entrants this year include former Navy Secretary J. William Middendorf II, who donated the trophy presented to the winner, Congressman Bill Alexander of Arkansas, and Harry Cordellos, 40, of San Francisco, who runs marathons in under three hours even though he is blind.

The sub-2:21 men are Camp (2:18). White (2:20), Charlie McGuire of State College, Pa. (2:18), Kevin McCarey of Villanova, Pa. (2:19), Dan Rincon of College Park (2:20), Air Force Capt. Chuck Burrows (2:20), Robert Clerk of Phoenixville, Pa. (2:20) and Scott Eden of Durham, N. C. (2:20).

Jane Killion, 29, of New York, who finished third among the woman in this year's Boston Marathon in 2:47:23, heads the female entry list of 453.

But the Marine Corps Marathon is primarily a race for recreational runners, the thousands who have no hope of crossing the finish line first but can win, anyway, just by finishing in a time that satisfies themselves.

This is a "people's race," and therein lies much of its appeal.

If you live in the Washington area, change are that people you know are running in this marathon. They are not behemoths, not supermen, but ordinary folks who have worked themselves into good enough shape to attempt something that is, for lack of a better word, heroic.

Even nonrunners can identify with these athletes, admire their fitness, respect their ambition and encourage them to have enough stamina to reach the finish line.

Ordinary people making the extra-ordinary effort to run 26 miles through a city full of fascinations. That is what Marathon Fever is all about.