Jack Fultz was relaxing as a friends apartment on Beacon Hill this idyllic afternoon, waiting the last supper of his week-long carbohydrate-loading diet, trying not to think too much about Mondays 81st running of the Boston Marathon.

The 28-year-old Pennsylvanian, a Coast Guard veteran who was graduated last summer from Georgetown University with a degree in economics, is staying with an old buddy, Steve Hoover, to save hotel costs. He drove up Friday from Washington, sharing gas and tolls with another friend.

"Its the cheapest way I had to get get here," said Fultz, who can confirm first hand that American's oldest and most prestigious road race does not pay expenses to anyone. He is the defending champion.

Boston" - as this tradition-filled and in some way tradition-bound celebration of sports for its own sake is universally known - does not pay a penny to any of the participants, who this year number 3,100 from 24 countries. It offers all who finish a bowl of canned beef stew.

The scuttlebut is that absence of expense money, to say nothing of under-the-table appearance fees for top names paid by some races, is the reason that Yale alimnus Frank Shorter has never run here. He is America's top marathoner, the Olympic champion of 1972 and runner-up to East German Waldemar Cierpinski last summer at Montreal.

But even without Shorter and Don Kardong of Seattle, who finished fourth in Montreal, the field is strong as well as deep. Boston - with 'Heartbreak Hill," the series of four agonizing taxing rises that begins about 18 miles into the 26-mile, 385-yard course - has a special lure.

It is old and famous. It presents a challenge that serious distance runners find difficult to resist. It attracts more spectators, hundreds of thousands of them, than any other foot race in America.

In the 1930s, the race was considered large with 250 entries. In recent years it has grown so fast that organizers have imposed qualification requirements - entrants must have run a marathon within the last year "there are now about 150 of them in the U.S.) in three hours or better. Women and men over 40 must have run in 3 1/2 hours or better.

For those with the necessary credential, it is worth plunking down the $3 entry fee to see how will and muscles stand perhaps the most greuling test in sports. It is even worse paying your own expenses.

"It may not have saved anything by not flying up because my car was towed Friday night," Fultz mused today. "I had moved it to what I thought was a legal parking space for overnight, but I gusee it wasn't.

Despite his misfortune, and the fact that he was bursting with energy for the exhausting physical and mental effort at hand, Fultz was in a mellow mood.

His thoughts were as serene as the scene on the banks of the Charles River a few blocks away, where worshippers of spring spent this Sunday with kites and bikes, frisbees and picnis baskets, sailboats and rowing shells, dogs and little children. It was a day for drifting, dreaming, rejoicing in the season.

Many of the contenders - and there are probably fewer than two dozen with legitimate title aspirations - undoubtedly spent the afternoon searching themselves, mulling private thoughts, visions, uncertainties, anxieties, hopes. One the eye of a big race this constitutes the lonelinest of most long distance runners.

But Fultz was loose, not the least bit uptight. He knows about the doubts, has experienced them before, but now likes to think positively, in terms of his own proven ability.

He has won the marathon before, and that is a great psychological advantage.

A yea ago, in stifling heat, Fultz strode out of anonymity, the first man to cross the finish line that is permanently painted across Ring Road, just off Commonwealth Avenue, in Boston's Back Bay section.

His time for the run that starts at noon in the picturesque Western suburb of Hopkinton and ends at the Prudential Center was one of the slowest in recent years: 2 hours, 20 minutes, 19 seconds.

A year earlier, then-Boston College student Bill Rodges covered the same ground in a record 2:09:55. He was aided by cool, pleasant weather and a wind that blew uncharacteristically toward Boston from the west, at the runners' backs all the way.

Last year, the temperature at the starting line pavement was in excess of 100 degrees, and what breeze there was came, as is normal, from the ocean, against the runners going east.

Fultz figures he gave us up at least five minutes last year to the heat that left many contenders retching and cramping, and scores of those who ran merely for self-satisfaction collapsed in pathetic dazed heaps on the roadside. They were picked up by ambulances (in the most severe cases) or by the "meat wagon" the panel truck that follows the foot foldiers, collecting dropouts.

Some experts think that Fultz will be lucky to finish in the top 10 this year, but he thinks experience could elevate him.

"As you get to a higher and higher level in competitive distance running, the specific of workouts becomes less significant and it becomes more and more a mental thing," Fultz said. "You know you're there, and you are your own source, and you are not surprised by the things you do. You no longer have to question them or prove them to yourself each time you compete."

That is why he was so tranquil today, as he eschewed the traditional marathon-eve visit to an Italian restaurant and help prepare spaghetti in Hoover's kitchen, the end of his week-long gorge of pasta, bread, candy, cakes, ice cream and other carbohydrate-filled foods.

"I've decided to do what I did last year - center on myself, run my best possible race, and see how that fits in," he said. "Hopefully, the No. 1 slot."