At the somewhat advanced age of 28, Renaldo Nehemiah is discovering what time will do. If the legs work, the arms won't; when the body is running, the mind is ambling. This is not stopwatch time, divided into hundredths, but significant time. When you earn roughly $1,000 a second, time will make you think.

Nehemiah, in his attempt to become what he was back when, has found that time has altered him both physically and otherwise, perhaps irrevocably. In 1982 he left an unparalleled career as the world's best hurdler to become a wide receiver for the San Francisco 49ers, an experimental journey into his ego. After four seasons as an unremarkable NFL player, he returned to track and field, only to discover over the last two years that while he might recover that form, he can't recover that person.

"I'm combating yesteryear versus now," he said. "I'm trying to mimic what I looked like. But I've gone through tremendous changes."

When Nehemiah races now, as he will in the Millrose Games at New York's Madison Square Garden on Friday and again in the Mobil Grand Prix Invitational at George Mason University on Feb. 15, he does not chase world records or other runners, or his considerable appearance fees, so much as he pursues an image of his former self.

That Nehemiah was self-described as arrogant, sleek and greedily unbeatable. The present Nehemiah is rich in both money and accomplishments, and would be happy simply to make the Olympic team that will go to South Korea for the Summer Games. An Olympics is the one thing he has yet to win, which is why he continues his comeback effort despite his difficulty returning to form.

Having gained and lost 15 NFL pounds, and taken equal doses of humility and maturity, Nehemiah finds himself a better person, if a lesser runner. His comeback has been only moderately successful by his former standards, which include world records in the 55-meter hurdles (6.82 seconds) and 110-meter hurdles (12.93 seconds). Nehemiah has not even approached those times, and longtime rival and two-time world champion Greg Foster has replaced him as the reigning man to chase, a state of affairs he greets with surprising introspection and patience.

"Now, I say, 'It's only seven seconds,' " said Nehemiah, in town yesterday making appearances to promote the meet at George Mason. "A few years ago I couldn't say that. Seven seconds used to be the most important thing in the world to me. I couldn't eat or sleep thinking about those seven seconds. I don't lose sleep anymore."

Rather, what Nehemiah seeks is a fitting close to a remarkable career. An Olympic medal would make his athletic life whole in the same way that he feels he has filled out off the track. Having transformed himself into a football player and back again, he had to take a close look at his mental and physical state, and learned some intimate details about himself.

Nehemiah is constantly asked when he will be back, to which he replies, "I am back." But the assumption is that he won't truly be back until he has overcome Foster. He has been gradually creeping up, losing to Foster in a 55-meter hurdles last week by .11 of a second. But he is also confronting the possibility that he might not ever make up that difference.

"I'm very reflective right now," he said. "At 28 I have an opportunity to understand myself as an athlete. Greatness is opinionated in a lot of things, like football. But in track it's factual. Now I'm coming back to close a chapter, by establishing myself again as one of the best hurdlers in the world, and hopefully the best . . .

"There's a part of me that would love to be the Renaldo of old. But every day I have to face reality, and every day reminds me that I'm different, that I might not have that burst and vibrant body of old."

To make up the rest of the ground, Nehemiah must grapple with two considerable alterations caused by his career change and new maturity, one in form and one in attitude. In learning to play football he ruined his hurdling technique, and in learning to play for a team he ruined his arrogance.

Technically, Nehemiah says he is still "atrocious" in his second season back. In going from 175 to 190 pounds and learning to run flatfooted on a field, he abandoned his form. When he returned to track and tried to run heavier, he found his body was out of kilter, causing a top-heavy hitch in his stride he still hasn't entirely corrected. The result is, while he may lead a race at the outset, he generally loses ground over the jumps. An Achilles' tendon injury over the last year also hampered his training, and he will devote most of this spring to trying to correct the flaw.

"If I can get this close while looking that bad, there's hope," he said.

Emotionally, Nehemiah has encountered more complex problems. With two world records that still stand, he finds his motivation is less, a problem he calls the "John McEnroe syndrome." With an interest in broadcasting, he is concerned with getting a future under way.

But mainly, having subjugated himself to a team concept for four years, he finds some of the ego that led to his records has disappeared. That is not all good. Nehemiah is considering talking to a sports psychologist to try to recover the missing ruthlessness.

"Now I have to turn that on," he said. "You have to be very selfish and driven in track . . . I don't think you can be as laid-back in this sport as I am right now. I don't think its logical to accept and be happy with a second-place finish."

Nehemiah finds himself thinking about other things at a race. He listens to the applause and savors his introductions as a world-record holder. The records he has little interest in chasing, a distinct change from his former persona, the one obsessed with the clock. "Now I'm like a flower child," he joked.

The result is a less certain runner, who is still in transition. The finished product, according to Nehemiah, could be just as fast as the old one, because he has retained his speed even if his technique is in question. But the new Nehemiah will enjoy it more, and gradually begin considering retirement.

"All I know is that when the gun used to sound, I was a bat out of hell," he said. "Now I'm thinking about four or five different things. People say they can't see me explode over the hurdles anymore, and that's true . . . Five years ago if I didn't win I was dejected all week. I was on a high and my ego was outside the arena, it was so large. I couldn't wait to shatter records. That was my thing.

"But you can't beat the clock. I tried. The clock always won."