OF THE 9,500 RESOLUTE runners who will line up this morning at the Iwo Jima Memorial for the start of the Marine Corps Marathon, almost 6,400 are first-time marathoners.

And when it's over, I suspect many of these 6,400 will be last-time marathoners, as well. A few simple calculations will tell you that.

Last year, for example, there were 9,100 entries in the Marine. If there are 6,400 newcomers this year, quite obviously there can only be a maximum of 3,100 runners entered today who ran the race last year, too.

Where have all the marathoners gone? Probably to shorter distances that place far less wear and tear on body and soul.

I am an avowed last-time marathoner, myself. I have even been somewhat soured on the whole race circuit ever since that day last spring in the Urban League's Home Town 15-kilometer run around the District when a sedentary spectator in Northeast yelled "Pick it up, fat boy."

Oh, I still run. It's the only way I know to have your cake and eat it, too. But I've pretty much stopped talking about it at dinner parties, and thoroughly put out of my mind any thoughts of running 26 miles in one day ever again.

I'm not recommending that course of action for everyone. If you enjoy it -- and obviously many runners do -- the marathon offers a rugged challenge and a great feeling of accomplishment at the finish line.

I'm only suggesting that you don't have to go that distance to derive all the wonderful benefits of running. Most medical people will tell you that getting your heart rate up to 80 percent of its maximum capacity for 20 minutes a day four times a week is more than sufficient to maintain cardiovascular fitness.

And many orthopedists also will tell you that the constant pounding of feet on pavement can lead to serious problems -- in the feet, the knees, the back -- way down the road.

Many of us found that out the hard way. Almost four years ago today, for example, three of my Washington Post colleagues and I entered the Marine Corps Marathon and wrote about the race in the newspaper.

Now two of us, Dave Kindred and Andy Beyer, no longer run. Beyer hurt his neck and traded in his running shoes for a 10-speed bicycle. Kindred hurt his back, which hurt his golf swing, which hurt his wallet and his pride.

This is the same man who once finished the New York Marathon, then took a taxi from the finish line to the Eastern Shuttle to get home. He now gets most of his exercise walking in the woods, looking for little white dimpled balls, which hurts his handicap, but not his back.

Colman McCarthy, who writes frequently about running and once did three marathons in the space of six weeks, and 12 in all, has not gone the 26-mile distance since last year's Marine. He stills runs 20 miles a week and snorts: "Don't call me an ex-marathoner. Frank Shorter once said you're not a true marathoner until you come back from an injury." Plainly put, Colman's got a bad knee that used to begin throbbing at 20 miles. Now it starts hurting after three or four. He says he'll line up at the start today, but does not expect to finish.

A year after we all ran the Marine, I still had an itch to go 26. So I entered the New York Marathon with three other friends from the neighborhood.

We had a delightful weekend, sleeping on the floor of my mother's house on Long Island, consuming a ton of spaghetti the night before the race, running through a funnel of two million New Yorkers on their best behavior.

And now, three years later, only one of my companions that day still does marathons. One runs a few miles occasionally on the weekend, and another has given it up completely. For him, the press of business, community and family responsibilities left precious little time to keep getting up at 5:30 a.m. to run.

There was another factor involved in my friend's decision to stop marathons. His wife was a nervous wreck on race day, particularly after she saw him at the finish line in Central Park.

"I had no problems at all when I finished the Marine the year before," said my friend, Bruce Davis of Haymarket, Va. "But I felt awful after the New York race. Even now, I look at the pictures of all of us afterward and I can't believe how bad we looked."

So Bruce promised his wife Janice he'd never run another marathon again, "unless of course someone sponsors me in the Honolulu Marathon," he says.

After failing to finish both the Marine and the New York, I, too, decided marathons should be left to people with skinnier bodies and knees that do not go click in the night.

Once, training for the marathon consumed me. I bought all the books, read all the magazines, kept my daily mileage totals in the official Jim Fixx calendar, experimented with bee pollen and loaded up on carbohydrates every chance I could.

These days, I'm happy to run four miles three or four times a week, a six-miler on Saturday morning, with a little tennis here, a little YMCA basketball there.

My marathon memories are not particularly pleasant. In the Marine race, I collapsed in a heap on Hains Point after 20 miles. In New York, I knew I was in deep trouble when the sun came out and the temperature hit 75. I pooped out after 18 on First Avenue and vowed that day to specialize in the shorter races.

I do, however, cherish some runs. The Super Bowl 10- kilometer through the French Quarter in New Orleans last June was delightful. London's Hyde Park in late August provided a true runner's high.

And so, too, did a run last Sunday -- three friends going eight miles on the C&O Canal towpath at Great Falls. River rapids on one side, casting fishermen on the other; the trees in full fall flame, a slight mist in the air and 80 minutes of pleasant conversation. Such are the joys of a last-time marathoner.