Because my parents were fine physical specimens I entered this life with a body that wouldn't quit.

At least, it never seemed until recently that my bone and sinew and liver and lights could ever fail me. I once ran my bike head-on into a bus and it cost more to repair the bus than to fix me and my Columbia. On boring afternoons I used to dive from the garage roof to the ground, or do forward rolls down our bare oak staircase.

From the perils of boyhood and through a misspent youth and 20 years in the newspaper game - which involves an awful lot of sitting with occasional periods of frantic activity - the engine output was demanded. Among the insults that the animal I live inside shrugged off were polio, beer, cigarettes, no regular exercise, awful eating habits and chronic lack of sleep.

"Guys like you die of hear attacks at 30," my friend the doctor told me when I was 25. At 35 I sat slumped, cigarette ashes cascading over my belly, which in turn was cascading over my belt, and listened to another doctor's calm and terrifying lecture on the error of my ways.

"I can tell by looking at you," he said. "You are a classic Type A. You do everything wrong and nothing right. I will bet you a thousand dollars that I can predict your blood pressure, lung function, cardiac capacity, reflexes and cholesterol level. You are 40 pounds overweight. You are on the verge of physical collapse. You're going to have a heart attack unless you have a stroke first. I'm going to put you through our mill and get some numbers that will scare the hell out of you."

I disrobed and was probed, tapped, blooded, scanned and prodded. I ran the treadmill and blew up the balloon and lifted the weights and stood on one leg with my eyes closed. Next day I went back to see the prophet of doom.

"Get out of here," he said. "There's nothing I can do for you."

"Is it that bad, Doc? How much time have I got?"

"Not more than a hundred years," he said. "Except for the weight you've got the numbers of a teenage distance runner. Perhaps I am in the wrong trade." Shortly thereafter he shifted his practice to North Carolina.

I went back to my bad habits, modified by a new assignment that took me outdoors a lot, paddling God's streams and sleeping under God's sky and puffing up God's hills. I thought all this healthful activity would keep ny numbers good.

It has not. As I rise 40 I am daily reminded that back there somewhere my body peaked and is stumbling down the backslope.

The first intimation of mortality was the vertigo.Occasionally, standing at a bus stop or in a queue, I suddenly find myself reeling. The moment passes almost instantly, but - especially when it happens at a cocktail party - the embarrassment lingers.

Then came the yawns. In terms of time and money spent, reading is my principal vice. Where once I used to read two books at a sitting I now do well to get through two chapters before nodding off. The New Yorker magazines, once my cover-to-cover delight, are piling up at such a rate that I am a John McPhee and a half behind, and I am troubled all week by the things I didn't read in the Sunday papers.

And the eyesight. I came into this world with 20/10 vision, meaning that where most people see a forest I could see not only the trees but the squirrels scapering around in them. I used to amuse myself by reading off all the lines of the eye charts and guessing at the printer's union bug. The eyes seemed indestructible; even a dart that pierced one retina and nicked the optic nerve left me with just a tiny blind spot, a problem only during navel inspections.

I hadn't seen an ophthalmologist in 20 years until I came up with a corneal scratch. As part of the routine he ran me through the charts, and I was shocked to discover that, squint as I might, I couldn't read the last two lines."Fine, fine," he said. "A solid 20/20. The astigmatism may bother you a little as you grow older."

"Astigmatism? 20/20? What can i do?"

"Be grateful," he snapped. "Your eyes are in fine shape for a man you age."

Defeat goes on.When I went to renew my driving permit the young person behind the eye machine kept showing me one card after another.I not only couldn't read the bottom lines, I had a little trouble with the upper ones.

"This is quite a test," I said. "Y'all didn't use to go through all this."

"You didn't use to be 40," she said. "After people reach a certain age we check closer."

There it was. I had become "a man of a certain age."

A semiconscientious effort to get more exercise - short of jogging, which was despicable even before it became fashionable - has had mixed results. I take some pains to walk or climb instead of ride; to push on instead of rest. And get some aches and pains in return. My feet have learned to hurt.

There was a comforting vagueness about the matter until I undertook to replace a rotted porch. I couldn't do it. That is, I could not, singlehandedly, lift into place the twins of boards I had handled solo seven years before. One of them stood me on my head. I had to go get ropes; I had to go get a jack; I had to go get my daughter.

"So what else is new?" said a friend of 50 when I tried to enlist his sympathy along with his help on the high parts. "You're getting older. You're also supposed to be getting smarter. Next time you want to lift a 16-foot 2 x 8, for instance, you might try grasping it in the middle."

He swung himself lightly onto a rafter.

"Pace," he said. "You have to pace yourself within your new limitations. When your energy tank starts running low, take a break until it refills. You want to get off your butt and hand me that sheet of plywood? I mean, if we're going to keep sending your son after beer at least one of us ought to swing a hammer now and then."

"Power," said a colleague, after watching me come second to him in a canoe race only because the third boat went in circles. "You're not using your power. You big ol" heavy boys are like that; you muscle along instead of channeling your power. You're wasting a third of every stroke."

"Pig," said a slender person. "You eat like a pig. Do you realize that if you lost 20 pounds you'd be 10 percent stronger?"

"Pedal," said Nachman, who has been being sensible since he had a stroke, and now looks 20 years younger. "Sell your van and get a bike. It will make a new man of you." Nachman sells bicycles.

"Pretend," said the old man who shuffles along the street and steals my elderberries. "Pretend you're 60, and then it will seem wonderful that you can still do all these things. I pretend I'm 100."

I like that old man.