At 19 miles, Kevin McDonald's legs and lungs told him that the ambitious early pace he had set might have been, as many suspected, suicidal. But he held on, resolutely refusing to look back and see what the opposition was doing, and won the second Marine Corps Reserve Marathon yesterday with the best effort of his late-blooming running career.

McDonald, 27, a health systems planner from Greenville, S.C., covered the level and scenic 26-mile, 385-yard course that started and finished at the Iwo Jima Monument in 2 hours 19 minutes 36.9 seconds. In 15 marathons since he started to run competively in 1973, this was his first time under 2:20.

Angry at being jostled as a pack of 2,655 runners started the "run through the monuments," McDonald went the first mile in less than five minutes. He set a furious pace the first 10 miles with luckless Marine Capt. Juan Garz, who fell out of contention when he had to find a bathroom in the 14th mile and was later struck by a car and knocked out of the race.

Phil Camp, 30, of San Diego, a Navy aviator stationed at Pensacola, Fla., was second in 2:20:09.1 after tailing McDonald the last half of the race through the District and Northern Virginia streets and parks. He closed to 75 yards during the 21st mile, in East Potomac Park, but could get no closer. Bothered by a nasty blister on his right sole, he finished two minutes behind his 2:13 career best, fastes of any of the entrants.

Local favorite Max White, 26, the 1972 Princeton graduate who now teaches at the Episcopal School and finished 22nd in this year's Boston Marathon was third yesterday in 2:21:32.4.

As was the case for many of the 2,323 finishers, that was a personal best time, achieved on a fast, fair, spiritually uplifting course in weather that was gloomy for spectators but perfect for runners-overcast, with temperatures in the 50s and winds of only 10 to 15 miles per hour.

McDonald went out too fast. He did the first mile in 4:58," said White, who, tactically, prefers to disregard the leaders and run within himself at a smooth steady pace of 5:15 to 5:20 a mile. "I thought, Great, he's going to fade," but he didn't. "He ran a great race."

"I had done a lot of speed work," said the 5-foot-11, 147-pound McDonald, who had been training 90 miles a week since his second marathon victory in Madison, Wis., in June. He would like to do more miles but can't because of chronic hip ailment; one of his legs is shorter than the other, causin a slight misalignment in the pelvis and more than 90 miles per week gives him muscle strains and spasms.

A native of New Jersey, he took up jogging in 1970, after a sore arm forced to quit as a pitcher for Villanova's freshman baseball team. But he never ran competitively until 1973.

"I was just doing it because I enjoyed the exercise and seeing the countryside," he recalled. "I had always played hockey and baseball and suddenly I found myself without an organized sport for an outlet. I started racing and surprised myself. I sometimes wish I had started sooner."

McDonald ran 2:20 in the Boston Marathon in 1975, finishing 25th the day Bill Rodgers set the U.S. record of 2:09:55. He ran 2:21 in the U.S. Olympic Trials at Eugene, Ore., last year.

"I wanted to run a sub 2:20 because I had never done that before." he said. "I was a little afraid I had used up too much energy too early and would pay for the last pace. I felt it at 19 miles. My legs got tired and I felt a stitch coming on my right side, so I slowed down.

"But I wanted to win and to run my fastest time. That's the only way I know how to run-aggresively."

Sue Mallery, 23, an Arlington native who now lives in Columbus, Ohio, and is doing graduate work in phsiology at Ohio State, was the top woman finisher in 2:54:12, good for 298th place overall. Last year she won the women's division in 2:56:33.

"I was really happy with my time." said the Washington-Lee High School graduate.. "I had a muscle cramp in my right thigh at 13 miles but told myself just to run through it. The people were sensational today, and that really helped. The hands and the spectators were fantastic.

The music supplied by the Marine Corps Drum and Bugle Corps at the Iwo Jima Monument, the Wakefield High School Band at six miles, and a 15-piece bagpipe band at the Lincoln Memorial 11 and 17-miles, psyched up all the runners. But Mallery got a special lift at the Kennedy Center from her older sister, Sandy Eggleston.

"She jogged along for a hundred yards or so - she's a swimmer, not a runner - and was telling me jokes," Mallery said.

The second woman across the finish line was Neli Jones, 24, of Unionville, Conn., who bettered by three minutes the 3:02 she ran two weeks ago in the New York City Marathon.

"I knew I was the second woman because the crowd kept telling me. It's great being a lady in these races," she said. "If I could have seen Sue it would have given me some incentive, but I never got close enough."

McDonald seized the lead almost as soon as a starter's pistol at the line and a booming 105-howitzer blast downrange sounded simultaneously, launching the pack on a run around the Pentagon, back past the Iwo Jima Memorial to Key Bridge, through Georgetown, around the Mall, the U.S. Capitol, and the Tidal Basin, along the river through East Potomac Park and back again to "the Iwo."

"Right at the start somebody was pushing and shoting, I gave him an elbow in the ribs. I was ashamed about that later, but it got me really mad," said McDonald, who gave up the lead only briefly to Garza in the seventh and ninth miles.

They were together most of the first 13 miles, never more than about 25 yards apart, often practically abreast.

"I didn't know who or how good he was," McDonald said. "We had a running conversation going between the seventh and eighth miles, but I thought we'd better stop before we ran out of air. We had introduced ourselves and talked about the course - he had seen it, I hadn't. I asked him his best time, and he said 2:48.

"You never know. That might have been his only marathon, and if so he could make a breakthrough, but I felt better knowing that, I didn't think he could keep up the pace."

Garza, a Texan based at Camp Pendleton, Calif., and running his third marathon, he was third here last year until dropping out at 22 miles with a groin pull, kept it up for 13 miles, through Capitol Hill, but ducked into Union Station when nature called.

That cost him a couple of minutes, but he was steaming along again until he got to the intersection of 15th Street and Independence Avenue, where a car pulled out unexpectly and knocked him to the pavement.

"I got up and took off again but got dizzy and fell. Somebody from the fire department picked me up," he said, after being treated for scrapes and bruises of the left side at George Washington University Hospital and released.

Meanwhile Camp, a helicopter pilot now training to be a flight instructor the niggled some Marines by running in a "Fly Navy T-shirt," took up pursuit of McDonald.

Gradually he closed th gap from about 400 yards to perhaps 100 at 17 miles and 75 at 21 miles. McDonald was reluctant to look back, but spectators told him Camp was approaching.

"I was really aware of him. I was getting tired. The crowd had been telling me pretty steadily '300 yards, 300 yards,' and suddenly somebody said '30 yards.' I started thinking second place wouldn't be too bad."

So it went through East Potomac Park, as McDonald's huge lead dissipated, seemingly inexorably, making him feel as weepy as the willows that line the river there. He allowed himself a glance at Camp as they left the park - "I only peek at corners because, psychologically, I don't want the other guy to think I'm worried about him" - and another as they turned off the 14th Street Bridge at 23 1/2 miles. The distance was back to 150 yards.

"I was sure I had him," McDonald said. Still the slightly sloping last quarter-mile brought doubts back. "It was all uphill," McDonald said, "and I thought it would never end."