On Sunday morning, if you find yourself staring at the flat bellies and lean-legs slide past - whether in the sprint of the front-runner or the lope of the I-just-want-to finish plodders - be prepared to hear the restless person inside you call out, "Why can't I do that?" It isn't a thin person or an ex-athlete that's within you trying to break out, it's the old self wanting a new definition of you. Few activities besides long-distance running offer a saner or surer chance for a person to get at his core - to strip away the mask by standing at the starting line of a 26-mile race to confront the reality sounded by the starting gun.

In the past few years, as I became less and less a follower of money-lusting, Lombardist professional athletes, I began to seek out long-distance runners. Of all our athletes, they alone seem to me to retain the purity of sport in the amateur sense, as in the Latin amator, meaning "lover." Those who have been running 50, 70, 100 or more miles a week for months on end to be ready for Sunday's marathon have not been motivated by the allures that compel most other American athletes: money, recognition or security. Marathon running has no leading money-winners, no Donald Dells hovering at the finish line with deals and no sportswriters hyping the latest champ. The day after the New York marathon - which saw the largest and strongest field ever assembled - the New York Post sports section gave four times more space to horse racing than to the marathon.

The payoffs and profits of long-distance running come in more enduring currency than money and fame. From what I have been able to learn in many extended conversations with men and women who run long distances, the delights of their passion touch all parts of the personality: body, mind and emotions. Running, as Washington amateur David Gottlieb says, is less a sport than a lifestyle. Except for a few clowns in Sunday's race - those who made a $10 bet in a bar the other night that "I can go 26" or the naive who think they are Rocky out for a sport of roadwork - all the marathoners are already well along the way to newer or fuller definitions of themselves, with Sunday's test affording a chance for still more physical, mental and emotional delight.

If people feel better because of long distance running, many talk also of "thinking better." They believe that 10 miles of cross-country "clears the head" as well as the arteries, that "ideas come to you" that you never get while sitting, and that running - the height of activity - can become the most contemplative pursuit. Edward Ayres, a veteran marathoner and publisher of Running Times, wrote in his magazine:

"For adults who perform highly complex tasks for a living, the simplicity of running may provide a welcome form of daily relief. For those whose jobs involve long hours of frustrating mental work, it often feels good to just "turn off" the brain and let the body take over for a while."

If people along the sidewalks see marathons as fetishes for masochists, runners know the accuracy of T.S. Eliot's remark - in a world of fugitives, anyone running the opposite way appears the madman. Marathoners cherish their madness: running from social pressures that glorify spectacular sports while dismissing the participatory ones, moving away from the false idea that self-discipline is dangerous and fleeing the illusion that satisfaction can be bought in a product or a vial. Unamuno wrote, "Unless you strive for the impossible, the possible you achieve will be hardly worth the effort." For many of those on Sunday, running 26 miles was an impossibility a year or two ago. They would not be trying for it at all, except that now they have had enough of life's "possibles." In the end, they agree with Jerome Drayton, who won the Boston Marathon last April: "When you go on a long, steady run by yourself," he told Runner's World, "your mind just drifts. I like this solitude . . . In the training, the mental activity, you become a calm and relaxed person. You have a joyous outlook toward life."

Perhaps spectators at Sunday's marathon will catch some of the joy of the runners. If so, that will be still another reason to be cautious in your onlooking. Indeed, you may not want to wait until next year for your first marathon. You may want to go a block or two in this one. No one will mind: Runners are understanding souls, and as you join them they know that you might as well take your first training run over the course where you will be going for 26 next year.