I'VE ALWAYS thought that people who book cruises to Bermuda or drive off to the same Rehoboth Beach cottage for two weeks every year would not have -- in a century past -- been among those souls who boarded covered wagons and headed for a new life in the West. And who could blame them? It was a dangerous and ardous undertaking.

But, human nature being what it is, there are large numbers of people today paying fairly hefty sums to put danger and hardship in their lives--and they call it a holiday. Vacationers, it seems, are divided between those who relish certainty and revel in nothing more than uninterrupted hours of dozing under a seaside sun--and their hardy opposites who shun cozy comfort in pursuit of the unknown.

They--this latter group--are the adventure travelers, a fast-growing breed. In times past, I think, they would have signed up first for the wagon trains pushing along the Oregon Trail.

Idle afternoons at a Paris sidewalk cafe'? Not for them, or anyway not by first choice. Their idea of fun is to load up a backpack with 40 pounds of food and equipment, heft it to their shoulders and tromp off into the highest mountain around--wind, cold and rain be damned. I know because I am one of them. Touring Paris has its delights, to be sure, but ----real joy comes in slogging up a rocky path where every slow step brings the thrill of a new vista and the satisfaction of having made it against the elements, at least this far.

To many (if not most), relaxing is collapsing on one's back; to us, it's moving out to some place new or something different, where uncertainty carries an element of danger to be conquered by brawn and brain, and one's capabilities are stretched and expanded. By day, drip in sweat under a scorching sun on a dusty Sierra trail; by night, shiver around the tiny flame of a portable stove when the high mountain air drops to freezing. Ah, that's the life.

Around the world, Americans are seeking adventure: by foot, bicycle, Jeep, canoe, rubber raft, sailboat, skis, horse, mule, camel, covered wagon (even today), sled, airplane, balloon or any other conveyance they can think of. Down the river, up the peak, into caves, across the desert, through the jungle, under the sea, in the sky, over the tundra, atop a glacier.

The reasons are many for the popularity of this kind of travel, but the one that explains it best for me is this: That segment of the nationwhich gets up and jogs three miles every morning isn't satisfied for very long sitting on the sand and watching the waves wash across their toes. Why else develop muscle and stamina if you're not going to use them? A day at the office, no matter how mentally invigorating, is hardly a physical test.

Much of adventure travel today (unlike that of the intrepid explorers of history--Columbus, to name one) is organized by tour groups whose leaders generally are skilled outdoors men and women who have completed the trip several times over. That tempers the danger--few travelers want to put their lives in real peril--but unforeseen hazards always exist in the wild.

Among the formal tour offerings, the possibilities for adventure are enormous:

Scramble onto a camel and share the life of Tuareg nomads, following camal paths from the mountains of Algeria to oases in the Sahara Desert (11 days, $1,675 from Paris, Visages du Monde).

Bicycle 200 miles down the Northern California coast, sleeping in tents under redwood forests (6 days, $447 including all meals, Backroads Bicycle Touring).

Trek with the Sierra Club for five weeks this fall to Tibet and Mt. Everest for views of the north and east faces (price yet to be determined).

Ride a four-wheel drive truck coast-to-coast in South America from Rio de Janeiro, Brazil to Lima, Peru via La Paz, Bolivia and the Inca ruins at Machu Picchu (9 weeks, $1,720, Encounter Overland).

Kayak along the cliffs of Alaska's Glacier Bay, exploring the fjords that harbor seals and puffin, bears and eagles (7 days, $675, Alaska Discovery).

Plunge through the twisting Hell's Canyon of the Snake River in Idaho, the deepest river cut in North America, aboard a rubber raft (6 days, $693, Echo).

Actually, an adventure need not be a journey to an obscure place under difficult conditions. I know a young Washington secretary from a close-knit family who had never been out of the United States. One day she flew off alone to France for a three-week tour on her own. As much as seeing the treasures of Europe, the pleasure of the trip came from venturing into the unfamiliar and discovering new strengths of self-reliance. A year later, she hitchhiked through the British Isles and rode into very proper London perched inelegantly in the cab of a huge cement truck.

Self-reliance is a valuable asset on any journey. A few years ago, six of us from the office rented two-man canoes and ventured an overnight paddle down the Shenandoah River to Harper's Ferry, W.Va. None of us knew anything about the river--or about canoeing, for that matter, except maybe to scoot across a quiet pond. In our perhaps foolhardy inexperience, we found adventure when we didn't expect it: rocks and rapids that capsized two of our three vessels, spilling food and gear into the water.

One of the group, a self-dubbed indoorsman, ended up clinging to a rock, caught between rushing torrents on either side. It was up to the rest of us, far from any emergency squad, to rescue him. I don't remember quite how we did it, but it involved getting as close as possible to him, throwing him a rope and holding out a paddle so he could edge himself slowly to safety. It could have been a moment that haunted his dreams for years, and I asked him about it recently. "It was," he recalls, "a lot of fun."

Adventure travel is hard to define precisely. A well-maintained mountain trail may challenge me, while a hand-over-hand climb up the smooth side of a deep canyon wallmay be well within your ability. For a farming cousin from the Nebraska beef and corn country on a first visit to Washington, adventure was sampling the strange foods of a fancy French restaurant. Give him credit, he braved escargots after only a little teasing.

There's probably an adventure in the offing if you keep asking yourself the night before: "Hey, what am I getting myself into?" When there are doubts, you know you're not headed for a cabin on the QEII or a room at the Homestead.

Another characteristic is movement. You go from one place to another by whatever means, usually requiring some muscle power that can leave you aching and sore at night, yet ready for more of the same the next day. I don't really understand people who pitch a tent and then sit beside it for a week. Putting miles under my feet, finding a new campsite by a different lake or stream every night, is my idea of camping.

And there is the possibility of danger, which I mentioned earlier, usually more fancied than realized. Imagined danger I've experienced any number of times in the out of doors: What's that sound? Are we lost? Actual terror was felt only for a moment, as I recall.

It happened one moonless night when I stepped away from camp a few paces--how shall I put this discreetly?--to dig myself a bathroom. Trousers dropped, I suddenly heard the campers farther down the lake shore banging pots and pans and yelling like crazy. They had been raided by a bear or bears, and, I was convinced, they were driving them in my direction--and I was in no position to get out of the way. Frightened (who wants a bear in his lap?), I began yelping, too. Fortunately, despite all the crashing about me in the underbrush, I was spared everything but the blow to myego. My hiking buddy has never let me forget that night.

One learns and grows on most adventure trips, but they have two other attractions. One is that you see any place you are visiting actively rather than passively through a tour bus window. You become intimate with the landscape, its climate, its inhabitants, and the journey is more valuable for it.

On a Maine windjammer sail a few summers ago, my wife and I relived all those classic stories of storm-tossed seas along the Atlantic Coast when a hurricane swooped down on us. Though we were anchored offshore in a protected harbor, the ship bounced and rolled through the night, the wind howled and splashed waves across the deck and the captain and his small crew, dressed in Down East slickers, kept watch at the wheel. It was alarming, yet welcome as a memorable event giving us a keen appreciation of the daring of wind-borne sailors.

The other attraction is the people who sign up for adventure trips. They know in advance they are in for some bumps, so there are no complaints when they occur. You don't fret about crummy accommodations when your room is a tent rolled up in your pack.

Once, on an nine-day rubber raft trip down the Colorado River through the Grand Canyon, our supplementary motor conked out in the midst of a turbulent rapids. The boat swung out of control, bashed up against a sharp rock jutting from the canyon wall, and suffered a long gash that immediately deflated the craft's right-front quarter-section. Spit out into quiet water again, the raft limped to a sandy beach. Our guide attempted repairs, but nothing could be done.

Five days from civilization upriver or four down and a mile deep in the canyon, we had only two alternatives. We could radio for a helicopter lift out or we could chance the raft's stability through the succession of rapids ahead. We chanced it, our group of 12 ranging in age from 19 (our guide) to almost 70 (two couples), accepting the raft's lumbering progress over the rest of the rapids as a matter of course. In the camaraderie of this shared adventure, it was amazing to see how deeply we came to enjoy each other's company.

Anyone whose discomfort level is easily reached might try a short adventure trip as an introduction to this kind of travel. On a family driving trip, make reservations for a day-long rafting trip, one in which you only ride and the guide does all the work, especially if your muscles are untrained. You will be splashed all day with water, as if someone were spilling buckets over you, but in the summer heat it feels good. Whichever of the states you are touring, you will see more of it in one day spent outside of the car and floating down a river.

Most good travel agents have access to catalogs of guided adventure trips throughout the world, and there are several on the market. Two good ones are "The Adventure Book" by Sobek's International Explorers Society (Frommer, 92 pages, $14.95) and "Adventure Travel: The Sourcebook of Outdoor Vacations in North America" by Pat Dickerman (Adventure Guides, 256 pages, $7.95).

Locally, sporting goods stores such as Appalachian Outfitters and Hudson Trail Outfitters are sources for information on raft trips and other outdoors adventures in the mid-Atlantic region, as are state tourist offices. After backpacking a few days with people who know what they are doing, you ought to be able to manage the next trip on your own.

If a particular activity interests you--cycling, backpacking, horseback riding, soaring--pick up a magazine aimed at that audience to find information on tours.

Other sources include the Sierra Club (530 Bush St., San Francisco, Calif. 94108); American Youth Hostels (1332 I St. NW, Suite 800, Washington, D.C. 20005); League of American Wheelmen (P.O. Box 988, Baltimore, Md. 21203); Mountain Travel (1398 Solano Ave., Albany, Calif. 94706) and the Adventure Center (5540 College Ave., Oakland, Calif. 94618).

In truth, adventure travelers are not entirely gluttons for punishment. You will find them sprawled out on their backs in mid-hike, probably grabbing a short nap to make up for the restless night spent on hard cold ground. They take time, too, to exalt at a view or simply to reflect. And after their adventure, they are as likely as anyone to soak in a hot tub, quaff beers in an air-conditioned pub or dine in a plush restaurant.

But then, after all, they have earned it.