Under the high sun at noon tomorrow, the nonsense starts. A fake gun will sound and about 5.000 cardiovascular systems will begin the pumpwork of pushing 26 miles along the roads from the Hopkins Common to downtown Boston. To call the Boston Marathon nonsense is to elevate it into the giddy air of which Plato spoke when he said that play can be the mind's most serious business.

What else is a run of more than 26 miles but earnest nonsense? Most other forms of nonsense in our lives remain only that - nonsense. And we know it. It depresses us and attacks us. Worse, it can make us depress and attack others, and lead us to wonder what kind of joke life is when we can so seldom push out of it for a few moments and take serious something that is hilariously absurd, like running 26 miles.

Few of our other absurdities can be laughed at, whether it is paying outrageously high taxes or consuming air. water and food laced with chemicals. But the nonsense of running over distances that would be better walked, biked or driven is one of the last refuges where a person can make a laughable fool of himseld and still be respectable.

Refuse to pay taxes and you are jailed. Fat nonchemical food and you are labeled eccentric. But be a running fool in the Boston Marathon and for a few hours you are relieved of the pain of having to make sense.

I was told the other day that for the past few monthe people have been offering to pay handsomely, even obscenely, for an official entry for Boston. These potential bribers - seeking to avoid the strict qualifying standards for Boston - have been rightly told that the price is paid not in dollars but in the more rare currency - desire.

I began desiring a day in the Boston sun about three years ago, when I realized that this was the only world-renowned sporting event that I would ever have a chance at. Kindly skeptics said I would never make it: "You would be better trying to race in the Indy 500." said one friend, "because you drive a car much better than you run." Although it was true that I had never run at all much less run well, I did know how to walk. That is the first principle: if you can walk you can run.

The second principle, which has seeped into my bones and muscles only after four marathons, is also simple: once you start running, think of everything but stopping: Think of your aches, your dreams, your family, your bills, the girl who just waved to you at the 10-mile mark, but blank out that illusory delight of stopping. Let the mind and body be lazy, but not at the same time. Chesterton end of the world. But the power of hoping through everything, the knowledge that the soul survives its adventures, that great inspiration comes to the middle-aged."

That is one of the beauties of the marathon: It is mostly for the middle-aged, the people from 30 to 80 who think they have been sufficiently kicked around by life to know a thing or two about the bruises but who really need the reminders provided by the marathon - that life is fragile and uncertain. Why else did Theodore Roethke write, "We learn by going where we have to go."

At Boston tomorrow, the best learning site comes at about 17 miles - the series of steep inclines known asd Heartbreak Hill. In other sports, you can psych out the opposition but hills are beyond psyching. Other sports are do-or-die but going up Heartbreak Hill it is die-and-die. For 17 miles, the runners will be giving until it hurts. Now we are forced to give until after it hurt.

If the most agonizing part of the marathon is the final six miles, the trying part of my training schedule has been these last six days. The mental doubts bark at me like dogs during a run, except I can't Mace them away. Did I run too much these past two months, the way you can be overgolfed? Or did I not go enough?

I know runners who go 22 miles just a few days before a marathon - their depletion run, they call it. It would be a madness run for me. Boston ends tomorrow, but it really began not in Hopkinton but in my bedroom last December and January when the most harrowing yards of long-distance running were covered: the movement in 6 a.m. darkness from the bed to the running shoes. What was the third principle: A yard in the bedroom in December is worth 10 miles of roadwork in April.

I spent the winter being moderately diligent about my morning "gruesomes," as they are called by the nonrunning athletes in my family. But I never had a run without my mind drifting into the anticipated thrills of the Boston Marathon.

I have been thinking lately that may be the wrong term: it is more the Boston Climax, a converging of all the physical and emotional forces that running out in Boston, when 5,000 runners look at 1 million spectators looking at 5,000 runners, is to relish a style of uplifting nonsense that makes much of life's demeaning nonsense that makes much of life's demeaning nonsense bearable. Without marathons, without Boston, where would we find flavor?