"Why are you doing it?" my wife had asked, shaking her head. The people at the office had just rolled their eyes.

But here I am at 3 a.m. with about 90 other hikers, getting ready to step out on a one-day walk from Washington to Harpers Ferry -- a little over 62 miles away. It's the Sierra Club's annual 100-kilometer hike along the C&O Canal for hard-core walkers who have yielded to the lure of the towpath, their own folly, or both.

There's some comfort in knowing that volunteers will maintain four food stops along the way, and that cooking facilities, showers and beds wait at a hostel in Harpers Ferry. But between here and that bed lie 62 miles of will, endurance and pain.

The pack takes off quickly from the Thompson Boathouse near the Watergate, and rounds the Lincoln Memorial. Walter, my walking partner, and I hold back in spite of the desire to keep pace. We've prepared for the Big Event by working up to 50 miles in 10-mile increments. We figure that if we can do 50 miles, what's another 12 miles, more or less?

We stick to our training and our slow pace. By dawn (Mile 9) we're completely alone on the towpath -- not a good sign. But by Mile 20 or so, we begin to overtake the early burnouts. At Mile 25 (Seneca), the first food stop, we overtake a large party of hikers. The stragglers and collapsers will never make it beyond Seneca because the rest feels so good, the coffee is warm and the food is plentiful and cheering.

At Mile 37.7 (White's Ferry), we overtake a group of the more serious hikers, leisurely munching Brie. We press on, confident because of our extensive training. But the trail is full of surprises. At Mile 42 my right ankle goes (sprained, as it turns out). My right ankle and right knee become blistering towers of pain.

At Mile 50.4 (Point of Rocks), we stop at a food station for bananas, iced tea and chocolate. Twelve miles to go. We reach Brunswick, Maryland, (Mile 57.2) as darkness creeps in. Five miles to go, but these are the worst. My ankle and knee are gone and our flashlights are weak (no extra batteries). Whenever I trip over a stump or step in a hole that I can't see, the agony is delicious. I wonder about crawling the last two or three miles.

At last, the final turnoff. How beautiful the sky has become over the mountains; bright white stars sparkling in the black expanse. Up the final quarter-mile hill to the finish line. And then, after 20 hours, I scale a score of remaining steps to victory.

Well it may not have been absolute victory, but in an event like this, distance is victory enough. HIKE-HO, HIKE-HO There are three main rules to finishing a walk such as the 100K: You have to train, you have to walk with someone and you can't stop too long to rest. It's not a casual outing. Participate in several long-distance training hikes -- few people finish who don't. You should also walk with someone to keep your mind off your suffering -- a radio is also helpful. Above all, you can't stop too long to rest. If you stop for more than a few minutes, it's all over. You tighten up and you never get up again. This year's 100K will be held May 7. Deadline for registration is this Saturday. There will be two training hikes of approximately 30 miles, this Saturday and next Saturday, April 30. For further information, call Sue McElfresh, 703/455-6312, or Joe Greene 301/585-1585.