Nine years ago, when the first Marine Corps Marathon was being plotted by a few, proud and inexperienced men in uniform, there were enough prerace worries to turn any leatherneck into a nail biter. And the main concern was not with permits, traffic control or race routes.

"I remember somebody asking, 'Suppose we give this marathon and nobody comes?' " said Col. Jim Fowler, the founding father of the Marine run, who shaped the idea of a race around Washington's monuments during an 11-month stay in a hospital where he was recuperating from a leg wound suffered in Vietnam. "It seemed like a natural but . . . this was the first race I'd ever organized."

About 1,200 runners showed up for the first Marine marathon in 1976. It seemed like a huge crowd to Fowler. So many men and women willing to pay to run 26 miles 385 yards.

On Sunday, when a 105mm howitzer thunders the start of the ninth annual Marine marathon at 9 a.m., nearly 12,000 runners will begin the long loop from the Iwo Jima Memorial in Arlington, through downtown Washington and back again. In nine years the Marine marathon has grown 10 times its original size, and only because organizers stopped accepting race applications at 12,000.

The running boom, which has grown exponentially almost everywhere in the United States during the last decade, may be fading somewhat as a media phenomenon. But as a participatory sport, there is no shortage of new recruits.

"There seems to be this annual supply of fresh meat for this thing," says Avery Comarow, a 39-year-old, Washington science editor who will be running his first marathon Sunday. More than 60 percent of those registered for the race, in fact, have never before attempted a marathon. "It shows there is an incredible appetite for this."

It might be easier to understand that appetite if the people who experience it could be squeezed into stereotypes. But look down the list of entrants in this Sunday's marathon and you discover a range in age and economic status too wide to tie any strings around.

Eric Stokes, a 6-year-old from Woodbridge, Va., is the youngest. Eighty-six-year-old Ruth Rothfarb of Massachusetts is the oldest. There are 193 accountants, two butchers, 30 FBI agents, seven ironworkers, four miners, 3,200 Marines and one lumberjack in a field of runners representing all 50 states and 40 nations.

Five brothers and sisters in the Russo family will be running their first marathon. A Marine colonel, Herbert Waters, will run with four of his sons, all of them Marines. Only Stanley Leventhal, a park ranger from Chevy Chase who is stationed at the Old Post Office in Washington, will attempt to set a Guinness record for running the race backward.

"It definitely gets people's attention," said Leventhal, who is trying to break the record of 4 hours 7 minutes.

You would expect someone who has spent six months in painful training for this marathon to have a ready, if not reasonable, explanation for the compulsion. But ask Comarow why he is running his first marathon at the age of 39, one year after triple bypass heart surgery, and you will hear an articulate man become temporarily monosyllabic.

"When people ask me why I'm doing this, I can only say, 'Because,' " responds a laughing Comarow, who knows no explanation will satisfy anybody who needs one. "That's what separates people who run marathons from people who don't."

On June 16, 1983, Comarow survived surgery to bypass three clogged arteries to his heart. After the operation, his cardiologist suggested he begin exercising to help the recovery.

But the doctor did not have marathons in mind.

"When I asked my cardiologist about entering this marathon, he said, 'I wish you wouldn't,' " said Comarow, a former track and cross country runner in college who has been overweight most of the years since graduation. Comarow does not think running a marathon is a particularly healthy thing for any human to do. But because of the operation, not in spite of it, he decided it was a challenge to accept.

"In my case, it's strictly something I need to do to get a personal albatross off my back," he said this week from his office at Science 84 magazine.

"I am no athlete. I want to make that real clear," said Comarow, who has two children -- aged 2 and 5 -- and a wife who is not exactly enthusiastic about his Sunday run. "This won't make me one. But it does mean that you endured. And I think that's important for a lot of people who push pencils."