"Dad, what's it's feel like? Are you hitting the wall now? You're really slowing down, Dad, Look at that guy - he must be 60 and he just passed you. And now a lady is coming up behind you. Dad, I mean you've really got to get it moving."

This was Jimmy McCarthy, my 10-year-old who met me at the 16-mile mark on his bicycle and was wheeling along in merry spirits as we came to the 20-mile pole past the bend at Haine Point. I had invited him to come along for the final 10 miles of the marathon.

Like a good liberal. I have raised this child to talk frankly with adults. But here he was, in the sweat candor of his youth, asking me what it's like to be hitting the wall.

It wasn't one of my better moments in father-son dialogue. Here I am I said to myself, hurtling headlong into the curved wall and I have this young inquisitive demanding a full commentary from me.

I still had an electrode or two left in my brain, so I caught exactly what was happening. I knew I had asked for this. I remembered all the times I had talked about the wall to Jimmy and his younger brothers. In the comfort of the living room and after a fine supper. They he warm in their pajamas and innocence and I'd hold forth: Boys, it's really something what I do in those marathons. I've yet to be flattened.

I told them about conquering the wall in New York at Heartbreak Hill, Beltsville, Satyr Hill in Baltimore, in last year's Marine Corps, my tales of middle-age triumph had the same ending: Corp. where you talk with the kids in school, tell 'em about Iron Legs, your old man who runs through walls.

So here was the kid next to me at 20 miles. He came out to see for himself. He knew what to ask: Do the cramps in your calves hurt more than your dizziness? Why don't you throw water over your head at the next water stop, the way Bill Rodgers does? Do you think this next hill is steeper than the last one? And always the wall - what's it really feel like?

I had a question for Jimmy: You want to be heaved in the Potomac head first or feet first? But I didn'y ask it. I was hobbling along in the survivor's shuffle. Talking would use up needed energy. I had gone out too fast: 37 minutes for five miles, 76 for 10. At 20 miles, I was deep in debt, I was paying for it.

Fatigue was so intense - I was beyond exhaustion into a state of helium pain where aches are blown into blimps of soreness - that just to do a 12-minute mile meant giving it my gutmost.

And all the while Jimmy looked on. Meat wagons disguised as ambulances came by for the lame and halt, and I probably would have jumped on except that this 75-pound boy on his bike was now carrying me along. That happens in marathons - one runner's emotions being the jumper cables that recharge the fading willpower of another. And now it was happening to me. He's only 10, I found myself thinking, and already I'm depending on him for life.

The thought came more in melancholy than self-rebuke. Poets and philsophers talk of life's circles: how fathers pick up their sons from the cradle and then, as grown men, the sons pick up their dying fathers from the sick bed to nurse them in the final days.

Here were the final miles of the marathon and I'm being nursed along. Jimmy knew by now that all kick had left me. All was timed now to collapse at the finish line, and perhaps salvage a bit of honor by breaking four hours. He knew last year I had done 3:26, so what was a cakewalk then was now all but a death march.

As for discovering the inner mysteries of the wall, Jimmy began to hit it at 25 miles. My legs, he announced, are killing me. A neighborhood darter, he'd never gone beyond five miles on his bike. But coming up hill to the finish line, he'd doubled his distance. That was his marathon.

We pulled each other in. With the agony of Sunday's homestretch already fading from both memory and muscles, I'll be back soon to gathering the McCarthy boys around me for some new tales of the wall. Except now, with hubris in check, I'll also be listening. The real story, not the heroic one, is out. The man is mortal. His legs aren't iron, they're flesh. And one day they will need nursing.