There was no loneliness among the long distance runners, 5,988 of them, who started yesterday's third annual Marine Corps Marathon. But for the last 10 miles, one man - Duke University medical student Scott Eden - ran by himself, far ahead of the crowd.

Only the two motorcycles and a jeep that served as trailblazing "lead vehicles," and a pack fo pestersome bicyclists, were near Eden during the last third of his 26-mile, 385-yard journey through streets and parks of the District and Northern Virginia.

The 25-year-old Richmond native completed the picturesque "run through the monuments" in 2 hours 18 minutes 8 seconds. He crossed the finish line, near the Iwo Jimma monument in Arlington, three-quarters of a mile in front of his nearest pursuer, Kevin McCarey of Eugene, Ore., who finsihed 3:47 later.

Jane Killion, 29, a commercial loan officer fron New York, was the top finisher among the nearlficer from New York, was the top finisher among the nearl1:55. That was 13:32 slower than she ran in finishing third in the women's division of the Boston Marathon in April, and did not place her in the top 400 overall.

Despite temperatures that climbed from 58 degrees when the starter's gun sounded at 9 a.m. to the low 70s later in the race - torrid weather for a marathon - Eden trimmed 1:28 from the course record of 2:19:36 set last year by Kevin McDonald of Greenville, S.C., who did not defend his title.

"I guess, on paper, I'd have liked it about 20 degrees cooler. But I've been training 90 miles a week in warm weather in North Carolina, so the heat didn't bother me as much as it did others," said Eden, who has spent the better part of the last eight years on the Duke campus in Durham - four as an undergraduate, one as a technician in the forestry lab, and three at the medical school.

Eden was on the front line, with the rest of the favorites, when a pistol and a down range 105-mm howitzer simultaneously launched the massive pack on the first leg of the race - an eight-mile loop down the six lanes of Jefferson Davis Highway to the Pentagon, around it, and back along Arlington Boulevard past the starting point and "the Iwo."

He was never more than a few steps off the lead, and broke away from a tight cluster of five runners on Capitol Hill. Or, more accurately, he sustained the ambitious early pace - the leaders ran the first 10 miles in 51:00, a 5:06-mile clip - while the cluster fell apart.

"I went out a little bit faster than I should have, and kind of burned the other guys off around the Capitol. I didn't really put a burst on them. They just fell back," said the 5-foot-6, 120-pound Eden, who had planned a 5:10 pace.

"I was amazed at how fast it happened. We were even going into the first hill by the Capitol, but by the time we got to Independence Avenue a couple of miles later, people were yelling, 'You've got 400 yards.' That's a lot of ground to pick up in two miles."

Seven runners - Eden, eventual third-place finisher Charlie Maguire of State College, Pa. (2:23:22), Bob Gray of Quantico, Jeff Peterson and Bill Albers of Fairfax, Dan Rincon of College Park and John Flar of Boston - were tightly bunched for the first eight miles, heading onto Key Bridge.

Two miles later - after a jaunt east on M Street NW, right on Wisconsin Avenue and left on K Street - the little cocoon of leaders was down to five, Gray and Peterson having dropped back. The others remained on each other's heels along Rock Creek Parkway, past the Lincoln Memorial and along Constitution Avenue to Capitol Hill.

But the cluster's last stand came at Union Station, just before the halfway point. Eden stepped ahead of Flora and the two of them pulled 25 yards in front of Albers and Rincon, who ultimately paid for the early pace and dropped out after 16 miles.

As if inspired by the band playing rousing marches on the lawn between the Capitol and the Library of Congress - one of seven bands that serenaded runners at strategic points along the route - Eden glided ahead at the 14-mile mark. By the corner of Maryland Avenue and 1st Street NE he was 35 yards ahead, and he opened a massive lead along Independence Avenue, between the Smithsonian and the Tidal Basin.

By the Reflecting Pool, his only competition came from the football players whose games he passed through. In East Potomac Park, where the weeping willows were hardly rustling and the river was calm, there was no one in sight behind him. He was running a private race.

"I just had to be concerned with not falling apart. There was the temptation to go for a great time, but my legs were feeling the punishment, and I knew I should just keep my rhythm. I have a tendency when I get tired to go up on my toes, so I just wanted to keep my feet flat and make sure I didn't 'die,'" said Eden. He got a cramp in his right side at 20 miles, but slowed the pace enough the final six miles to finish without extreme discomfort.

As a measure of just how dominant Eden was, McCarey - who went out moderately, was 25th after 10 miles, but just kept up his steady pace to a second place finish as the rabbits fell back - never saw the leader the last half of the race.

"Nobody wwas going to catch Scott today," said McCarey, 24, a native of New York and 1976 Villanova graduate (he was an 8:35 two-miler there) who is now part of a two-year Olympic training program for the marathon and 10,000 meters in Eugene.

"I didn't pick it up. I was just maintaining, and I couldn't believe how everybody else started dropping. I saw Charlie (Maguire), who was second, at about 19 miles I caught him at 21 and went by him at 22."

Maquire, 26, an academic advisor at Penn State University, lost 75 seconds when he stopped to change shoes after getting a nasty blister on his left instep. Later he got a similar blister on his right foot and finished the race painfully, with bloodstained socks as his red badge of courage.

Local favorite Max White, 27, the former Princeton cross-country runner who now teaches math at Episcopal School in Alexandria, finished seventh in 2:25:56.

Eden's time was 2:20 faster than his previous career best, a 2:20:28 effort that won the Greensboro (N.C.) Marathon 13 months ago.

Winner of six Atlantic Coast Conference championships in distances ranging from two to six miles while he was a Duke undergraduate, he now has his eye on the 1980 Olympic trials at Eugene in the marathon only.

He tried to train for both the marathon and the 10,000 meters in 1976, overextended himself and wound up missing the trials altogether. "I tried to run through some injuries," he said, "and they ran through me."

Now he has decided to concentrate his efforts on the longer distance.

"I just don't have the speed to do anything nationally in the 10,000. I may not in the marathon, either, but I have a better chance at that. I have a different schedule in med school, so I can train reasonably well again, and I have much better endurance than last year," said the third-year student, who expects to specialize in internal medicine and running injuries. "I hope I can get down to 2:15 this year."