WITH WASHINGTON'S Marine Reserve race coming up in November, hereare five do's and one don't for the elderly runner comtemplating a first marathon:

Train hard for several months.

Believe what you've read about "hitting the wall."

Believe what you've heard about the last six miles.

Believe in the collapse point formula.

Don't believe in your fantasies.

Hold back on the downgrades.

I ran my first marathon in June, ripe with the wisdom of my 54 years and bursting with confidence. I didn't finish. Worse, I had to publicize my failure to a regiment of inquisitors.

All first-time marathoners talk incessantly about the coming ordeal. Their families and friends are bored by their runologues, but once the terrible day is past, all these folks who didn't care about what was coming are nastily eager to learn how it went.

I signed up for the Governor's Cup marathon in Helena, Mont., with two friends a quarter-century my junior. At lunch one day a middle-aged non-runner interrupted our training table talk to snap at me irritably: "I sure hope you have a strong heart."

"We'll find out," my friend Jerry Holloron said.

There were other somber notes in the pre-race period. For example, the course map sent to entrants showed checkpoints at 3-to-4-mile intervals. These were labeled "air stations."

But I was sure I could go the distance. I had been running for more than two years, was averaging eight miles a day, had competed in two hard seven-mile races and had given myself a final pre-entry test of 14 miles. And I had a plan. I would start like a tortoise at 10 minutes a mile, and if that pace got me to the 20-mile mark in good shape, I'd finish like a hare. I scoffed at the cliche that the first 20 miles of a marathon are warmup for the real race. I knew that if I could run 20 miles, I could run 26. In my fantasy I saw myself circling the quartermile track where the race was to end, jauntily lifting my cap to the applauding crowd. I was Walter Mitty in shorts.

The day before the race, my friends and I drove over the course. It seemed long. From the starting point in a forest east of Helena it took us an hour to drive back to town. And the area seemed unfriendly. Many No Trespassing signs. At one ranch entrance, where hospitable Westerners normally hang legends like "Diamond Bar" or "Lazy K," we read "Keep Out."

But our misgivings melted with the butter of our carbohydrate-heavy last supper (I had spaghetti and strawberry pancakes) and at 7 the next morning four busloads of athletes headed for the starting line. At 8:15 the gun sent 103 of us off on a gravel road through a cool canyon toward Helena.

I felt wonderful, followed my plan and soon had no company. Once in a while, when the canyon opened up, I could see runners far ahead, but mostly I was racing the way I like to train - by myself, at may pace.

A few miles into the run I passed a parked station wagon with coolers and picnic baskets on the tailgate and canvas chairs with people in them scattered about. It was like an autumn scene in Princeton the day of the Yale game. A voice called, "Nineteen miles, 385 yards to go."

I was still running well at the 10-mile checkpoint, and exchanged pleasantries with the people at the aid station there. I didn't know it, but this is where I began coming unglued. At 10 miles on the Helena course you break out of the green and shady canyon, exchange the gravel surface for macadam, and start up the first of two long hills. For the next dozen miles there is no shade. It is desert, and on this day the thermometer was climbing through the upper 80s.

The first couple of miles in the sun seemed like good ones, though, because the runners up ahead began coming back to me. Two were young women who had come all the way from the University of Colorado for this race, only to have it end for them on the first hard hill.

The hill probably ended it for me, too, because at the top I blundered. A steep hill, going down, is seductive, and I let it pull me along, thinking I could make up a little time without a lot of effort. A bit later, when my knees began to hurt, I knew I should have resisted temptation. In a marathon, each foot hits the ground about 20,000 times and each stride sends a shock to the knee. The pounding of a fast run downhill added to the trauma.

But I came off the hill in good shape, I thought. I was pleased to pass two men just before the 13-mile aid station, and started up the next 2 1/2-mile-long hill in high spirits.

Before I topped it, i hated that hill, and at the 17-mile aid station I took a rest.Back on the road, a car slowed beside me and the driver asked if I wanted a life. I said no, thanks. He shook his head in disbelief. "Okay, pal; good luck," he said. I figured I wasn't looking my best.

From the aid station the road dropped gradually to the valley floor, and this stretch produced another bad sign. My hips and knees had begun aching and I had thought the gentle downhill would relieve them. It didn't. It was as hard to run down as up.

The road stretched ahead in a shimmering ribbon. I could see no runners. The heat was terrible. The next aid station was four miles away. The next mile marker confirmed my suspicion that I was in trouble. "It had to be 19," I thought. "I must have missed one." It was 18.

But I still had hope, and I got new energy when I caught a young runner and he praised my condition as I went by. Then my son and his girl friend materialized in a car, cried out that I was "looking good," stopped and poured water over my head. He said later that I'd looked all right on the move, but at a standstill had swayed badly.

Nevertheless, I passed mile marker 19 and mile marker 20. Then I hit the wall.

Marathoners describe this phenomenon in different ways. For some it's numbness of the legs, or dizziness, or nausea. For me it was pain. Pain in the hips, in the thighs, pain in the knees, pain in the calves, pain in the feet.

The hips were the worst. They were like unoiled hinges, and I thought I could hear the bone of the ball grinding against the bone of the socket. To finish a marathon, they say, you have to run through the wall. Halfway through the 21st mile, I knew that I couldn't.

In a way I'm glad I stopped. If I hadn't, I'd have missed the sights from the sheriff's car that took me to the finish. Napoleon's road back to France after his defeat at Leipzig must have looked like this. Limping, stumbling, wobbling, bending, tottering, weaving or propped against trees and utility poles were the survivors of the chipper, chatty crowd that had assembled in the woods 3 1/2 hours before.

Despite all, 80 runners finished. My friend Tom Newmann was one of them. Holloron wasn't.

Out problem, Holloron and I agreed, was that our training hadn't been tough enough. We had been running regularly for a long time, but had been pointing for the marathon for only a few weeks. We had read but not trusted the "collapse point" formula - that at three times the distance you run in training, your body exhausts in energy reserves.

Neither of us had collapsed, but no doubt we would have. We had been running eight miles in training; Newmann had been doing 10. I dropped out of the race at 21 miles and Holloron at 22; Newmann finished. On us, the formula looked good.

For two days after, all parts of my legs ached. But I was rerunning the race in my mind and planning for next year.

I've been running 12 miles daily and I figure I am moving my collapse point out to 36 miles - well beyond the 26 miles, 385 yards of the marathon.