A FRIEND watched astonished one afternoon recently as I gulped down six jelly doughnuts and a cream soda. "Why are you eating all that stuff?" he asked.

"I have to," I said. "I'm in training. Shall we go to lunch?"

I explained that I recently hiked for a week with a group of about 50 people who are walking across America on a 15-month backpacking trip called Hikanation. They will be coming through Washington this Wednesday. I discovered that the real challenge that faces these brave hikers as they cross this great country is not the challenge of the bone-jarring, muscle-wearying miles of road that stretch out before them. No, the challenge is the junk food they encounter as they pass through the general stores, gas stations, diners and cafes in the small towns that dot the landscape of this nation.

I thought I could meet the challenge and fact it down, saying, "whole wheat" and "organic carrots," but I was wrong. It was too tough for me. I found out quickly that, when you walk into a small-town general store at the end of a day of carrying 50 pounds for 15 miles in the freezing rain, you do not crave crunchy apples and lean meat and grapefruit juice. The only things that really hit the spot are Ho-Hos and Ding-Dongs and Reese's Peanut Butter Cups.

I arrived at my first Hikanation camp eating like a bird from my healthful, if meager, supply of backpack food -- instant oatmeal, beef jerky, whole grain crackers and some freeze-dried dinners. After the first full day of hiking 13 miles on moderately hilly terrain with a heavy pack, I was too tired to eat dinner. I set up my tent, brewed a cup of tea and went to sleep.

By the middle of day two I was feeling stronger and so was my appetite. I stopped at a grocery store, bought some Good and Plentys, some Sugar Babies and a quart of chocolate milk.

When I got to the gas-and-grocery at Eddyville, Ill., the shelves had been nearly emptied by hikers who had beat me to it. "You folks sure do eat a lot, the proprietress said to me. "Why, one boy bought a blueberry pie and sat right down on the steps and ate it all." Then she eyed my purchase. "What're you going to do with that big package of cinnamon rolls?"

"Oh," I said with what I hoped was a convincing smile, "I'm going to share it with lots of friends."

I wasn't out of her parking lot before I had it out of the wrapper, tearing pieces off with my fingers and stuffing them into my mouth. As I walked back toward my tent, eating as though I'd just gotten out of a concentration camp, a long-term hiker fell in beside me. "I see you're getting right into the spirit of things," he said.

The next day it snowed, and the cold weather perked up my appetite. At the end of the day in Herod, Ill., I found a grocery store, bought a poung-and-a-half can of beef stew ("Serves four") and a one-pound can of tomatoes. I heated the whole business over my camper's stove and, to my surprise, ate every bit of it. I was sorry I hadn't bought a Hershey bar for dessert.

By the end of the week I was eating like King Kong, and the resemblance didn't stop there. I'd popped the waistband button on my trousers. I was having a hard time zipping up my parka.

On my last morning I walked into a cafe ("our specialty: fresh catfish and hot corncakes") in Marion, Ky. When the waitress said, "We're all out of biscuits and gravy," I suspected most of the other hikers had already been there. When I ordered four eggs over easy, ham, double order of toast, orange juice, choclate shake and peach cobbler and she didn't so much as blink, I knew for sure they'd been there. While I ate, another hiker joined me, ordered a breakfast similar to mine and ate it with relish. "Gosh, it's almost lunchtime," she said, and ordered a cheeseburger with fries.

I told her I was leaving but hoped to join the group later. We belched goodbyes at each other, and she added, "You'd better stay in good shape."

I'm doing all I can.