Athletes teach us how to age. Their bodies do things ours never will, but they also know the limitations long before the rest of us do.

They know what it means to fall apart. "It's a terrible experience," said Bill Rodgers, the marathoner. "We get it all squished togther. A lot of the feelings people have at 65, an athlete has to cope with at 25, 35, 40."

Rodgers is 35 now and coping with those feelings about not being able to do what once he could. "I don't want to project illusions about myself," he said. "I don't think I have that much."

He ran his 38th marathon in Boston last Monday. That's four miles short of 1,000. This one hurt because he wanted to finish in the top three and qualify for the world championship this summer in Helsinki. And it hurt because his feet went numb. He ran on, thinking about retirement, saying to himself, "There is no reason why I should be feeling this much pain. With most marathons there isn't this much pain. I can run 10 kilometers and 20 kilometers. I don't have to do this anymore."

He finished 10th in 2:11.58. For him, it was not a quality time. So, as he sat on the edge of a national guard cot in the basement of the Prudential building, surrounded by ghoulish looking faces wrapped in aluminum foil capes, he talked about running only one more marathon, the Olympic Trials in 1984.

Now, he says, he'll run one more this year--perhaps New York, perhaps Fukuoka, Japan--and then the Olympic Trials. He has no illusions about winning a gold medal in Los Angeles in 1984, no visions of setting a world record. He simply wants to run a quality time one more time.

"I think I can crack 2:09," he said. "I hope to join the 2:08 club."

The world record, 2:08.13, was set by Alberto Salazar in 1980. Rodgers' personal record is 2:09.27 (Boston, 1979). "It's very legitimate to say my marathons have been subpar the last two years," he said. "Psychologically, it hurts a lot. I ran three miserable marathons. I was proud of the 2:11:08 in Melbourne (in 1982). But you have to do 2:09. If I don't do it this fall, then I'll have to admit the handwriting is on the wall."

In February, he missed two weeks of training because of a back problem (he hurt his shoulder trying to close a hotel door). The week before Boston he was home with a cold, taking medication. "They're writing him off and he ran 2:11.58 after being sick," said Greg Meyer, the winner, a friend.

Jeff Darman, president of a local sports promotion firm, said, "There's no question he has the ability to have been second, conceivably first if he had the strength early on to be in the strategy of the race."

Last year, Darman said, "when people said, 'Rodgers can't run a decent race,' " he made up his mind and set a personal record for 10 kilometers. "In 1978, I ran 28:36," Rodgers said. "I didn't break it until last year. I ran 28:26. Now I've got it down to 28:15. I focused on it for five weeks. I have to think I can do the same thing in the marathon."

But he has to do things differently. Boston taught him that. Aging is only part of the problem. In the last decade, running has come of age. As a four-time winner of Boston, a four-time winner of New York, Rodgers became a symbol and a marketable commodity. After years of running for nothing, he ran for money, the logical thing to do. Now he knows what cashing in can cost.

"I blew Boston," he said. "A lot of it was my fault. I have no one to blame but myself. I was greedy a little bit."

In the months before the Boston Marathon, he ran 10 races in 12 weeks, including one marathon. He earned about $46,000 for races and clinics, more than $50,000, including commercial endorsements. It wasn't worth it. "I could have run half of them and been successful," he said.

Once, Rodgers could run quality races in succession, sometimes on successive days. "He just can't race as much," Darman said. "He's not washed up. He's still an international class runner. He can't be the best but he can be among them. In a tactical race who knows? But he doesn't have the body of a 26-year-old any more. He can't race quality race after quality race."

Meyer said, "It's saying no, it's being focused like he used to be. He needs to train with people. At this stage, it's hard for him to do all the hard work by himself. He's going to have to gamble with injury. Most of the time he's tired from travel. He's going to have to be tired from training."

"I've always been the type that did too much," Rodgers said. "It was not purely financial."

This time, it caught up to him and he could not catch the competition. "It was stupid," he said. "I underestimated people like Benji Durden . . . When Benji made his move (in Boston), I wrote him off. I thought, 'Benji's insane today.' I thought, 'Ron Tabb hasn't run a good marathon. He'll come back. He's just being an idiot.' Maybe it's a sign of being at it too long. The only reason I finished is that I kept telling myself, it's my last marathon."

Durden led for nine miles and finished third behind Tabb.

With his blue eyes and blond hair, Rodgers has an innocent Billy Budd look that belies his competitiveness. "I was on the stand and I was 10th," he said. "I can say nice things about people. But I can't hide how wretched I felt. I was furious I ran so badly. I was envious, too, that Durden and those guys are going to Helsinki."

He thought he knew what he had to do to finish in the top three. He thought he was determined enough. He wasn't.

Once, not that long ago, they called Rodgers the marathon man. "I really believe I can crank one more out," he said.

When you are accustomed to a body that accepts the most punitive marching orders and a brain that tells you anything is possible, it is so hard to acknowledge fragility. The line between omnipotence and impotence is as fine as a tendon. "Athletes don't realize they have Achilles' tendons," he said softly, wryly. "Even though they should."