Lt. Joe Jenkins finished the recent Marine Corps Marathon with blistered feet, a sore hamstring muscle and second thoughts about whether he really was in proper condition to run the long race.

Some doctors, also, are having second thoughts about the merits of marathon running for the masses. Others -- although enthusiastic about the sport --are concerned that too many "overuse" injuries are cropping up because of improper training before a race.

By one measure, Jenkins, 25, did very well. Although it was his first such race, he finished 34th in the field of 9,753 with a time of 2 hours 31 minutes 19 seconds.

But shortly after finishing, the Marine from Camp Pendleton, Calif., collapsed and was carried on a stretcher to a first aid tent set up to cope with emergencies.

The medical staff at first concentrated on his feet. Then, as his worried wife and mother watched, Jenkins became chilled and cramped. He spent the afternoon wrapped in a blanket, as three bottles of intravenous fluid dripped into his dehydrated body.

"The question is, 'Is it worth it?' " said one doctor, as Jenkins and dozens of others sought medical assistance, or assurance, after the 26.2-mile race. Sometimes the end of a race looks more like a battlefield than an athletic event.

The difference, of course, is that the "wounds" generally are self-inflicted and, although they often are temporary, many might have been prevented. Medical experts warn that the enemy is usually the runner's own enthusiasm in training for and running the race at a pace that can go beyond his or her capacity.

The relative risks and benefits of running, particularly at marathon distances, are undergoing renewed debate in the medical profession. Most doctors endorse running at moderate levels, but there now is a recognition that nagging injuries can be the all-too-frequent result for those who overdo this form of exercise.

"Running will remain the most practical and economical way for large groups of people to get aerobic fitness, but we have to make it as safe and healthy as possible," said Dr. Lyle Micheli, director of the division of sport medicine at Boston's Children's Hospital. "We've had problems with the excesses . . . I'm concerned about the emphasis on marathon running."

"I think marathon running, which I do by the way, is a little bit akin to mountain climbing: if you like it, it's a lot of fun, but it is a little dangerous," said Arthur Siegel, an internist at the Brigham and Women's Hospital in Boston. "You can do it safely or you can do it recklessly."

"Everybody has a risk-reward ratio" in terms of exercise, said Gary Guten, a Milwaukee orthopedic surgeon who came to Washington to run in the marathon. After citing a litany of possible problems, his personal conclusion is that the "risks are minimal compared to the rewards."

"Let's not throw the baby out with the bath water," said Richard Keelor of the President's Council on Physical Fitness and Sports. "Let's not put down marathoning. It's marvelous to see more than 9,000 people lined up there. With proper training a lot of injuries may not happen."

BENEFITS: As in any sport, the marathon has attracted advocates, including doctors who make absolute claims that long-distance runners live longer and don't die of heart attacks.

Most doctors are much more cautious. Ronald Crystal, a pulmonary expert at the National Heart, Lung and Blood Institute who ran in the marathon here, divides the benefits into "definite" and "questionable" categories.

It is "definite," he said, that "energy expenditure is increased" and weight is better controlled. Serious runners also tend to make healthy changes in their life styles, sleeping more, smoking and drinking less, Crystal said.

There also are positive psychological benefits for runners. In addition to the general feeling of well-being that comes with exercise, Crystal said that the "runner's high" may have a physical basis in the release of endorphins, or natural pain killers, in the body.

But it remains difficult to prove that running eventually reduces the risk of heart attack. Although a number of studies have shown that regular exercise can reduce the risk, Crystal and others said that it is difficult to take self-selection out of the equation. "The catch," he said, "is who is the person who takes it up?"

Assuming that exercise does have cardiovascular benefits, which is the generally accepted wisdom in medical circles, there is still the question of how much is enough.

"For healthy individuals, 20 to 30 minutes of sustained exercise three or four times a week promotes cardiovascular fitness," said Siegel. "Strenuous and prolonged exercise beyond that, such as long-distance marathon running, carries some additional risk without any proven benefits."

"People want more than cardiovascular benefits," countered David Brody, medical director of the Marine Corps Marathon and an orthopedic surgeon who heads a George Washington University runners' clinic. While 20 miles a week might be sufficient for fitness, he notes that marathon runners run two, three or four times that amount.

"The target organ is the head," said Georgetown University cardiologist Samuel Fox. "If you help people feel better about themselves they will continue to do something which in time may benefit their health."

RISKS: The hazards, particularly at marathon levels, include sudden death from heart failure, pulmonary damage from inhaling pollutants in the city or exercise-induced asthma, mild to life-threatening heat reactions, hazards from the cold, injuries to muscles, joints and bones, automobile accidents, psychological problems and more minor concerns such as runner's trots (diarrhea during a race). Women may have additional problems with menstrual irregularities.

There is general agreement that the most common injuries are to the muscles and bones subjected to considerable stress from the constant pounding on pavement.

Dr. Richard Schuster wrote in a recent issue of The Runner that almost all distance runners develop injuries at times. "Authorities are calling this a new kind of epidemic. Surveys indicate that some 60 percent of those who run 30 miles a week and 80 percent of those who run 60 miles a week develop injuries."

Micheli said he sees a "real increase in injuries when people get beyond 50 miles a week in running. It's like a medical barrier. All kinds of problems develop, from tendinitis of the heels to irritation of the soles of the feet and knee problems."

Brody, who recently completed a study of 4,000 runners who sought treatment at his clinic, agrees that injuries are proliferating. He has found that injuries are most frequent among beginners and among those who seek to increase their mileage.

But Brody believes that the injuries -- in his study, 40 percent were in the knee -- are for the most part minor and treatable.

No one knows whether the long-term wear and tear from long-distance running can itself create permanent damage to the bones and joints. The arguments go both ways, with doctors citing autopsies of longtime runners whose joints look like those of a 20-year-old and others that look like those of someone who might have been 200.

Experts do agree that the risks can be minimized with the common sense precautions of moderation -- in getting started, in increasing distances, in judging how much is enough for a given person and in considering running in combination with other sports.

But there are no firm guidelines as to who should not consider long-distance running. Many doctors suggest that individuals with a family history of arthritis might not be good candidates for marathons. Others are concerned about the possibility of long-term damage to children who run.

"We do feel the musculo-skeletal system -- the bones and joints -- is relatively softer and there is more of a risk of chronic injury" to children, said Micheli. "More and more kids are entering marathons. We don't know how safe it is for kids to run these distances . . . They may get injuries that may not become apparent for 20 to 30 years."

For those who start running after age 35 or 40, many doctors suggest a physical examination to check for underlying heart disease.

Otherwise, it's a matter of individual judgment and goals. "People, in their enthusiasm for getting in shape, tend to forget the fact that our bodies don't all respond the same way," said Robert Leach, chairman of orthopedic surgery at Boston University School of Medicine. "Each of us has to learn his own limits."

He advises a distinction between running and racing. "The majority of injuries I see in my office are from people who race."

Fred Allman Jr. of Atlanta, president of the American Orthopedic Society for Sports Medicine, believes the marathon "should definitely be reserved for a segment that not only gets a psychological lift but also has the anatomy and physiological setup to allow them to do so."

Most of the problems that runners encounter can be related to errors in training and therefore can be reduced if the runner proceeds with moderation and caution, said Allman.

Keelor stresses the need for stretching and flexibility exercises before and after running and Guten urges runners not to increase their mileage more than 10 percent a week. Micheli, who found in a study of 53 stress fractures that 70 percent resulted from training errors, urges the beginning runner to build up from brisk walking alternated with slow jogging four times a week.

"The worst danger is overenthusiasm, social peer pressure and a runner's own compulsions. Why do it so much?" asked Milvy, who admitted to his own craziness in attempting to run in two marathons a week apart.

"I pushed myself too far," Jenkins, the Marine lieutenant, acknowledged a day after the race. Despite the fact that his preparation had slipped in recent weeks and he hadn't been feeling well, he still flew across the country, determined to enter. "I just wasn't prepared to run. I hadn't trained enough."

His bad experience has not diminished his desire to run in marathons. Other runners, however, are cutting back.

Anne Ballard, 36, public information director at the National Institutes of Health, ran in three Marine Corps Marathons and vows not to "run races ever again."

After running for six years and building up to 60 miles a week with no apparent problems, she developed a minor foot problem that temporarily stopped her training. She came back too quickly, pushed herself in a race and recently underwent lower back surgery for a ruptured disk she believes was associated with her long-distance running.

Ballard said she plans to turn to less abrasive forms of recreational exercise, such as bicycling, swimming, brisk walking, in combination with a little jogging -- in moderation.