More is not always better when training for a marathon. Over- training ruins just as many, if not more, marathon efforts than heading to the starting line being under prepared.

"Runners with specific time goals are most at risk for over-training," said Dr. John Cianca, medical director of the Methodist Health Care Houston Marathon. "People who train for a marathon simply to finish are less likely to over-train. It's runners who say 'I want to finish the race in under three and a half hours' who tend to overdo it."

A combination of increased mileage and decreased rest is the typical over-training recipe. Over time, this leads to a buildup of stress in the back, legs and feet and can set the stage for an injury that can knock a runner out of the race.

So how do you know when enough is enough?

"The most helpful indicator is simply how you feel," said Cianca. "Constant fatigue, lingering aches and pains, irritability, weight loss and recurrent viral infections can all be signs that you're putting too much stress on the body."

For some runners, 50 miles a week might be too much. Others, including world-class runners, log more than 100 miles a week without running into trouble. The key is rest. "As your miles go up, so does your rest time," said Cianca. "Many runners who increase their mileage to train for a marathon find they can no longer get by on seven or eight hours of sleep. They need more sleep to recover from each workout."

There are also ways to rest even when your head is not on the pillow. "Cross-training can be very effective in helping runners strike a balance between fitness and fatigue," he said. "Participating in another sport once or twice a week allows runners to stay fit, but gives them a break from the impact associated with running on the roads."

Cianca says the best cross-training sports for runners are water running, cycling and in-line skating.

Cianca, in his first year as marathon medical director, is an assistant professor in the Department of Physical Medicine and Rehabilitation at Baylor College of Medicine in Houston. He is also medical director of the Human Performance Center in Greenway Plaza and a consulting staff physician at The Methodist Hospital. Cianca specializes in the treatment of sports-related injuries and will coordinate all medical aspects of the 1998 Methodist Health Care Houston Marathon, scheduled for Jan. 18, 1998.