While slots in the New York City marathon are difficult to come by, for thousands of injured would-be participants, the race is over before it begins.

The marathon is 11 days away, but city doctors specializing in sports medicine are already seeing a run on appointments. Physicians say strenuous pre-marathon regimens, such as 20-mile training runs, can take a toll on the body.

Knee pain is the most common complaint of long-distance runners, the marathon’s medical director, Lewis Maharam, said. Dr. Maharam, who has a sports medicine practice on West 57th Street, estimates that his business increases by an average of 35% in the two weeks before and one week after the annual 26.2-mile race.

This year’s ING New York City Marathon, sponsored by New York Road Runners, is slated for Sunday, November 5.

On the day after the race, Dr. Maharam schedules no appointments. “People come in, and they get a number,” he said. “It’s like a bakery line, stretching down the hall.”

The director of sports medicine at NewYork-Presbyterian Hospital-Columbia, William Levine, said the marathon is “not very good for your body.”

“It’s above and beyond what most people need for good fitness,” he said. “It’s a couple of standard deviations from the norm. People do it as a personal challenge.”

Tendonitis and cartilage tears resulting in knee pain are common marathon-related injuries, according to Dr. Levine. “There are patients who don’t really recover, and end up with chronic orthopedic problems,” he said. Earlier this week, Dr. Levine said he treated a patient who ran last year’s marathon and has been experiencing knee pain ever since.

Also seen are more serious problems, such as stress fractures in the hip, thigh, and calf bones, Dr. Levine said. He recalled operating on a marathon entrant who fractured a thighbone in the middle of the race several years ago.

Long-distance runs can cause injuries beyond muscles and joints, physicians said.

Before they begin training, prospective marathoners should undergo cardiovascular screenings, a New York- based cardiologist, Lori Mosca, said.

Runners may also experience an arrhythmia, or exercise-induced asthma, or even a fatal heart attack, Dr. Mosca, the director of cardiology at NewYork-Presbyterian Hospital-Columbia, said. “Many people who get significant injuries have warning signs prior to the race,” she said, noting that chest tightness, heart palpitations, limb numbness, and nausea can be signs of serious heart-related ailments. “It’s absolutely critical people pay attention.”

In rare cases, dehydration during marathons lead to “temporary decline in kidney function,” a kidney specialist who teaches Columbia University Medical Center, David Cohen, said. Occasionally, long-distance runners will also see blood in their urine, and should seek medical attention, Dr. Cohen, the director of Columbia’s kidney transplant program, said.

Some doctors also warn of the possibility of overhydration, which can cause dangerously low blood sodium levels.

“I’ve been surprised by the extent of the minor aches, pains, bruises, chafing associated with training for the marathon,” a first-time marathoner Lee Schneider, 53, said. “Your body really does get beaten up.”

Mr. Schneider, a toy manufacturer from Melville, N.Y., said he spent the past five months training for the November 5 race.

Doctors stress that runners should cut back on long-distance runs two weeks before the marathon, and avoid trying new stretches, shoes, or sports drinks on race day. At the marathon’s official “fluid stations,” water and “Lemon-Lime Gatorade Endurance Formula” will be available. Along the course, there will also be “medical aid stations,” where injured entrants or those experiencing discomfort can stop for help.

In the days leading up to the marathon, entrants should take special care to stay healthy and injury-free, a marathon spokesman, Richard Finn, said. “That means staying away from people who are coughing, and being sure not to trip on anything,” he said.

Each year, more than 10,000 accepted applicants fail to make it to race day “due to injuries, other mishaps, or just cold feet,” Mr. Finn said.

There are 93,000 marathon applicants, about 50,000 of which are invited to participate in the race. Slots can be obtained by participating in nine qualifying races in a calendar year, by guaranteeing a $2,500-plus charitable donation, or through a lottery. Between 35,000 and 37,000 people, ultimately, taking part in the race.

While about 98% of those who start make it to the finish line, last year 741 runners did not. “It could be an injury, an upset stomach, or that they’re not running well and say, ‘I’ll try again another day,'” Mr. Finn said. “It could be blisters or cramps. You’re putting your body under a lot of stress.”