The Marathon.

The phrase itself has an aura, a romantic fascination, even for those who never quite have realized that in running it refers to a specific distance: 26 miles 385 yards.

The mystique has become part of the language. We hear of "marathon bargaining sessions" and "marathon matches." The marathon has a recognition factor, a history, a lore that no other running event enjoys.

It conjures up images of Olympians. Of lean, straingin men like Yale alumnus Frank Shorter who at Munich in 1972 became the first American ever to win the gold medal. Of ancient Greeks and the roots of physical culture, the celebration of strong mind and strong body.

Images of laurel wreaths, nonmaterial accolades, and hundreds of thousands of spectators lining the roads from suburban Hopkinton to Copley Square in downtown Boston on a magical Monday each April.

And images of Pheideppides, the legendary Greek who is reputed to have run from the coastal town of Marathon to Athens in 409 B.C., announced the victory over Persian legions, and promptly died - having, as someone observed, literally run his heart out.

Now, in a soaring movement that astonishes the few old hands (old feet?) at the event, the marathon has become the fashionable distance for tens of thousands of men and women who have gotten the running religion.

If the running boom is a phenomenon of the 70s, as has been well documented, the marathon must now certified as an official boomlet.

Consider these facts:

Until 1966, there were never more than 12 marathons in the United States in a year, never more than 150 runners finishing in under three hours. As late as 1970 there were only 73 sanctioned races, with 812 finishers in three hours or better.

Last year 197 marathons were run with more than 25,000 different participants, 4,000 of whom bettered three hours. This year nearly 250 certified marathons will attract between 50,000 and 60,000 official entrants.

Until two years ago, the only U.S. marathon with a field of more than 2,000 was Boston. Last year there were at least 10, led by the Mayor Daley in Chicago and the New York City with approximately 5,000 each.

The U.S. Marine Corps Reserve Marathon in Washington, though only two years old, drew 2,655 starters. The Honolulu Marathons attracted 3,050 - 95 percent of whom finished - and a record 619 women.

The Boston Marathon, the oldest continuous race in America, normally attracted a maximum 250 runners until 1965. It will be run for the 82nd time tomorrow with some 4,700 official entries - 1,600 more than last year, despite rigid qualification time requirements.

The January issue of Running Times listed more than 75 sanctioned marathons in its circulation area (the East, South and Midwest), including eight on one weekend on the East Coast alone.

"The number of people running marathons has doubled each of the past three or four years," says Virginian Ed Ayres, editor of Running Times. "Mathematically that can't go on indefinitely, but it has made the trend quite conspicuous."

A new quarterly, The Marathoner, debuts this week with a press run of 40,000. The publisher of parent Runner's World says 10,000 subscriptions have been sold in six weeks and projects paid circulation of 75,000 whithin 18 months.

The cover story of the current Pshychology Today compares the mental processes of world class and recreational marathoners, while last week's Sports Illustrated featured the first of two excerpts from a new book by Dr. George Sheehan, the noted cardiologist and philosopher of long distance running who concludes that for a new marathoner, once is not enough.

A recent TV movie, "See How She Runs," starred Joanne Woodward as a beleaguered mother and teacher who reasserted sagging self-esteem by completing the Boston Marathon. Though inauthentic, it drew a large audience in prime time and doubtless motivated many armchair athletes to start jogging.

Undeniable, "marathon chic" has arrived.

At first glance, this is astounding. Barely a decade ago, the marathon was regarded as a feat contemplated only bu superhumans, masochists, and the odd Ethiopian. There was little inkling it would become the cat's meow for thousands of suddenly fitness-minded, middlesclass Americans.

But on closer inspection, marathon mania is a logical second phase of the running boom. Given its longtime glorification in the popular mind as testing, and character revealing, it was natural that the marathon should become the supreme goal of rapidly multiplying new generations of recreational runners.

Long distance running is healthfully addictive, and many of its no-longer-lonely disciples consider the marathon their ultimate fix. For those who progress to it from jogging, a self-improvement that requires diligent effort and tolerates no short-cuts, the effort becomes a quest. The marathon represents the possible dream, the reachable star.

"I ran my first at Atlantic City in 1972, after I had been running two year," recalls Dave Theall, 46, of McLean, president of the D.C. Harriers. "It was agony and ecstasy. I really hurt, my knees ached, but I was euphoric."

"It's the obvious goal for every distance runner, simply because he's heard about the marathon, about Pheidippides and Boston, before he even got into it," says Jeff Darman of Washington, president of the Road Runners of America, who personally thinks newcomers would do well tro experience shorter races before attempting 26.2 miles.

"To some extent it's become a kind of macho thing, too. The marathon is a big undertaking, but it suddenly has become considered the thing to do to prove you're a distance runner," continues Darman.

"Something like 40 percent of the starters in last year's Chicago and New York City marathons were running their first race. That figure staggered me. But no matter what some of us who have qualms say, the marathon is always going to be held up as the event to aim for.

"Once you get into running it's a natural progression, to meet a challenge that had seemed inconceivable."

"For anyone past the jogging stage, it's upthere - the mountain to clim. It has a magical pull," says Bob Anderson, editor-publisher of Runner's World.

"It is literally the only event in which the average man can participate alongside the world's best. It's a Walter Mitty dream that is attainable. A sandlot baseball player can't be in a World Series game with Reggie Jackson, but an average runner can be in the same race with Frank Shorter."

Anderson thinks we have seen only the toe of the sneaker so far. That is why he founded the Marathoner as both a comprehensive chronicle and a "how-to" guide for first-timers.

Darman, while he would like to see the marathon de-emphasized, concedes that its popularity is self-reinforcing.

"At one time you practically had to travel across the country to enter a marathon, but now there's one in every hamlet, so it's more inviting to try," he says. "With more marathons there's more publicity and media coverage, and people see others they can identify with doing it. When they see the guy down the block or from the office doing it, it doesn't seem such a superhuman feat anymore.

"And the big explosion in women's marathoning is yet to come. The running boom is reaching women, but they're a couple of years behind the men. When they catch up, they'll be running marathons in similar numbers."

Marathoning is also being spread by its own considerable literature. Since running tends to appeal to the thinker, the introspective aesthete, it has been the subject of some superlative writing.

Surely part of the challenge that makes the marathon so appealing is overcoming the phenomenon called "hitting the wall" - the devastatingly sudden physical and mental exhaustion often accompanied by nausea, light-headedness, cramps and other tormenting discomforts, that generally strikes about 20 miles into a race, making the last six seem infinitely longer and forbidding.

Physiologically, "the wall" has never been fully explained. It is thought to result from depletion of glycogen, the fast energy sugar that most humans can store in amounts sufficient to run 18 or 20 miles. Thereafter, they must rely on much a slower-burning energy source, fat, which can be painful.

Evidence indicates that most world class runners are so finely conditioned and attuned to their bodies that they can avoid "the wall" completely, however, by adjusting their pace to finish without exhausting their energy reserves.

"The object for a competitive runner," says Jack Fultz, the Georgetown University grad who won the Boston Marathon in brutal heat in 1976, "is to keep from dipping into the reserves for 16 or 18 miles, then to judge how much you have inside you and divvy it up so you take you last effective step at the finish line."

"It's an interesting trade-off," says Ayres. "The fact that the prospect of 'hitting the wall' looms over every marathoner adds to the fascination."

Recreational marathoners are generally not so preoccupied with speed that they try a pace as deadly as that of Pheidippides.

"The saving grace is that most people are going into marathons with the goal of completing them, rather than attaining a given time," says Darman. "They are into running rather than racing, which is totally different from the old days when marathoners were mainly competitive runners who kept increasing their distances."

This is an important distinction, fundamental to understanding the modern marathon madness. In any given race, there are only a handful of runners with any realistic hope of winning. That is of no concern to the masses, who are competing only against their own expectations. For the recreational runner, who by nature may be non-competitive, the goal is set not by the stopwatch but by the spirit.

"The typical American is looking for ways to fulfill himself or herself, and marathoning presents a psychic high, a test of both physical and emotional fitness," suggests Bostonian Kim Prince, producer of a TV documentary called "Run, America, Run."

"There's a feeling that finishing a marathon reaffirms your aliveness. It's an intensely personal experience, a happening. The person who finishes in 4:14 can feel just as courageous and satisfied as the one who finished in 2:14. Simply finishing affords a self-respect that's hard to find today."

Kenny Moore, the Olympian, once offered this observation: "After every experience, it's natural to reflect that you might have done better. Only after a marathon can I say I have given everything. Because of the enormity of the attempt, the cleansing of the pain, I can sit, even stiff and blistered, and know a kind of peace."

Any individual who completes the marathon within his own self-imposed standard can safely say the same thing, and know the same peace.