The Nation's Capital still lacks a major-league baseball team, but this weekend Washington joins a select handful of cities with an honest-to-goodness big-league marathon.

At 9 o'clock Sunday morning more than 3,000 runners in the second annual Marine Corps Reserve Marathon will leave the Iwo-Jima Memorial in Arlington for a 26.2-mile tour of Crystal City, the Pentagon, Georgetown, Watergate, the kennedy Center, the Lincoln and Jefferson Memorials, Capitol Hill and points in between, before returning to the Iwo Jima Memorial via the 14th Street Bridge. On their way, they will be encouraged by clapping, cheering crowds.

As Bostonians have known for decades, and New Yorkers have learned in the past two years, there's a mystical kinship between a city and its marathoners. Boston can mark the passage of the years by the legendary champions - Clarence DeMar, Tarzan Brown and Gerard Cote - who have dominated its event. Whitehaired grandmothers cheering Old John Kelley, now the darling of the crowds at 69, can remember him chasing Cote up the Newton hills as a young man in the 1940s.

So too now in New York, where close to a million people turned out last month to watch Bill Rodgers repeat his victory effort in last year's first citywide marathon. Someday New Yorkers will no doubt argue the merits of Rodgers and Frank Shorter with the same kind of passion now reserved for Babe Ruth and Joe Dimaggio.

The kinship extends beyond the front-runners to the ordinary men and women who tackle the distance to prove something to themselves. in Boston children hold out their hands to be touched by runners - any runners - as if such a touch carried magic. Whatever pleasure spectators gain from watching marathoners is given back in the form of encouragement. Everyone who has struggled to finish a marathon can remember a shout or quiet word of praise that kept him going when he wanted to quit.

There have been marathons in Washington before, of course, but they have not attracted the numbers of runners, nor the spectator interest, to deserve comparison with Boston or New York. The chilly Washington's Birthday Marathon in Beltsville in February lures only spectators "with a life-or-death interest in one or more of the runners," as one account put it. Even last yeat's initial Marine Marathon, which drew more than a thousand runners, was chiefly known to the public for having created a traffic jam on the route to National Airport.

This year it's a whole new ballgame. The Marines have laid out a new route that goes through some of Washington's most scenic locations, and they have more than doubled the number of runners, inviting comparison with the massive fields in Boston and New York. There are no Olympic superstars in Sunday's field, but at least two top-class athletes are scheduled to compete: Phil Camp, a Navy pilot from Pensacola who has recorded a 218 marathon, and Alex Kaisch of Morgantown, W. Va., who finished second to Shorter in a recent race in Chicago.

Veteran marathon-watchers find two distinct pleasures in an event such as Sunday's. There is "the race," the fleeting passage of a handful of top-class competitors running furiously with intense concentration at a pace most people would describe as a sprint. Then, for an hour or more, follow "the people," men and women with little or no hope of victory in the traditional sense who are pursuing private goals of their own. Humanity in all its variety, beauty and frailty is represented here: old and young, male and female, tall and short, handsome and ugly. Most will hope to complete the course within a specified time - perhaps fast enough to quality for the Boston Marathon.

More than 3,000 runners had entered when officials closed off entries on Monday.Of that number, about 900 were active-duty and reserve military personnel, and more than a hundred were women. Almost two-third of the runners will be trying a marathon for the first time.

The entries include John Pianfetti, a 70-year-old from Charleston. W. Va., and David Levvi, a seven-year-old Rockville boy who will be running with his father and brothers. The roster also contains a 53-year-old woman and a 13-year-old girl. Some of the occupations represented include 34 accountants, 141 lawyers, 89 consultants, 34 computer specialists, 92 engineers, 56 salesmen, 10 artists, 45 physicians, four clergymen, 10 bankers, two firefighters and one cook.

Some tips on marathon watching:

If the weather promises bright sunshine and temperatures in the 70s, don't tell your running friend he's got a "perfect day" for the event. It may be perfect for your fall [WORD ILLEGIBLE] , but most runners would prefer temperatures somewhere in the 40s ro 50s.

Cheering, clapping and personal encouragement are considered good form in spectators. "Looking good!" is a better salute to a bedraggled runner than "Hang in there!"

Although the Marines will have official refreshment stations, there is nothing wrong with offering water, ice, Gatorade, orange slices or even (good naturedly) beer to runners. Offers of hard liquor or cigarettes are rightly considered insulting.

"Hitting the wall" is a phrase used to describe the body's utter refusal to run any farther, a phenomenon that commonly occurs during the last six miles of a marathon. If your running friend tells you he hit the wall at 22 miles, don't ask if he ran headlong into an Agriculture Department building by mistake.

If your running friend has hopes of qualifying for the Boston Marathon and he finishes a minute or two short of his goal, don't say brightly. "Well, at least you finished . . ." Friendships have broken up for less.

Finally, if you find yourself wanting to run next year's Marine Corps Marathon after watching this year's, don't be alarmed. It happens in the best of families. WHERE TO SEE THEM

The table below shows the approximate arrival time of leading runners at key check-points along the marathon route, based on a world-class pace of 5 minutes per mile. Best viewing areas are in the vicinity of the Lincoln and Jefferson Memorials.(TABLE) MILE(COLUMN)LOCATION(COLUMN)TIME 0(COLUMN)Iwo-Jima Memorial(COLUMN)9:00 3(COLUMN)15th & Joyce Streets. Arlington(COLUMN)9:15 6(COLUMN)Pentagon (River Entrance)(COLUMN)9:30 9(COLUMN)Key Bridge at M Street(COLUMN)9:45 10(COLUMN)Kennedy Center(COLUMN)9:50 11(COLUMN)Lincoln Memorial(COLUMN)10:00 12(COLUMN)12th & Constitutlon NW(COLUMN)10:05 13(COLUMN)U.S. Capitol(COLUMN)10:15 15(COLUMN)3d & Maryland SW(COLUMN)10:30 16(COLUMN)Jefferson Memorial(COLUMN)10:40 20(COLUMN)Hains Point(COLUMN)10:50 22(COLUMN)Tidal Basin(COLUMN)11:00 24(COLUMN)14th Street Bridge(COLUMN)11:05 25(COLUMN)George Washington Parkway(COLUMN)11:11 26.2(COLUMN)Iwo-Jima Memorial(COLUMN)11:20(END TABLE)