Little doubt exists that today's Marine Corps Marathon has been conscientiously trained for by most of those in the 5,000-person field. I know of a number of runners who, more than a year ago, began increasing their mileage. They watched their diets and did hill work - all for today's run.

These runners will find that the work has been worth it. The Marine Corps Marathon, over a monumentally scenic course, has already developed a flavor of its own since it began three years ago. It is both a runner's marathon - good times are attainable here because, except for a knob or two, no hills are on the course - and a sightseer's marathon for those who want to take a hard look at themselves under pleasant race conditions.

For the beginners who has yet to go the full distance, the mental strain is in running headlong into what is intellectually uncharted. No one has yet described adequately what happens to the mind during a marathon. Physically, the body will be fine for 16 or 17 miles. It will balk at the ordeal of running that far without rest, but the body's argument with the mind will have been a losing one up until then: the willpower to run is able to defeat the physical urge to stop.

But then at about 17 or 18 miles, the arguments of the body begin to get to the mind. Suddenly, the mind caves in. It sees the total rationality of stopping.

Pain and fatigue in the body are now so dominant that the mind begins to desert the consciousness. It joins the clamor raised by the muscles and bones that the run is absurd. However appealing the intellectual reasons many have been that persuaded you to enter the race, these reasons are either no longer remembered or else are seen as bizarre.

The experience of brain desertion can't be trained for. It is not much talked about, either, among runners. It is almost too personal to be put into words. When runners talk about how they 'die' during the the last six or seven miles, they tell of their legs getting rubbery or the loss of carbohydrates. To tell of the mind dying is to admit a vulnerability that is almost too private a weakness to put on public display. Yet it happens.

Those who earn their livings by using their wits or who are paid according to their intellectual alertness the ones who find marathon brain desertion the most unsettling.The one reliable faculty - the mind - is suddenly gone. One's identity is stripped.

If any help is available, it is likely to come either from the crowds along the route or other runners who may not have been stricken so severely. I have no research other than my own experiences - and these are limited to just six marathons - but the race in which losing my mind was the least chilling was the one - Boston - where the crowds were the largest. Where it was the most chilling - Beltsville - was where the crowds were the sparsest.

Just as the body can draw energy during a race from water and perhaps food like oranges, I think it is possible that the mind can absorb the intellectual calories provided by the enthusiasm of spectators.

When the critics of running say that we are a mindless herd, we know how right they are. What we need to be careful about is being sure that at least we can reach the 17 or 18 mile mark mentally alive. If we blank out before them, it is going to be hard day.

Ask a friend to meet you at 18 miles and call out a few amiable words. Skip the oranges. It doesn't matter now. But a friend's words can serve as a brain scan. An electrode or two of energy may be enough to keep you going.