SHE TOLD her boss she was taking a hike. "Everybody thinks I'm a little bit crazy," says Jean Cooley, for whom the call of the wild was so strong that she quit her job as a legal secretary in D.C. to spend six months completing the Appalachian Trail.

"I don't have a career I feel committed to. Backpacking means a lot to me. It's exciting, builds character, and it's definitely 'off the beaten path,' " says Cooley, 31. "I think my job would have driven me crazy these past two years if I hadn't planned to do this."

Hiking is an avocation for Cooley, who lives in Bethesda and typically earns money for a trip by working for six months to a year beforehand. This is her second six-month trip on the 2,100-mile trail; weather and tendinitis kept her from finishing on her first trip. This time she's going to close it out, plus hike a number of side trails for a total trip of more than a thousand miles.

"It's like being on two different planets," she says, comparing trail life to her existence in the workaday world. "Time stretches out on the trail -- I have more experiences in a single day than in two weeks back home. I've met so many people, seen so much beauty -- the woods, the animals -- so many things have happened. It feels like a year since I left."

Cooley actually began her trail adventure in early March, and has since hiked through much bad weather, including late-season snow. Now in Maryland, she reports that the weather is cooperating, and the trail is getting crowded on weekends. She has seen deer, skunks, wild turkeys and raccoons along the way.

The other night, she says, "I was alone when I saw a sliver of light I couldn't figure out, and then the moon broke very suddenly over the top of a mountain. I just sat there with my mouth open. The trail . . . Virginia has been absolutely gorgeous."

Tens of thousands of people hike at least part of the ridge trail through the Appalachian range each year, and each year more than a thousand begin at the trail's Springer, Georgia, terminus with plans to cover it all the way to the top of Mount Katahdin in Maine. About a fifth of them will finish, taking four to six months to do it, many toting six-packs or wine up the steep last segment. (The Appalachian Trail Conference in Harpers Ferry sells an "A.T." jackknife with a corkscrew for the celebratory bottle at the top.)

Cooley found the going difficult at first. The Great Smokies, where she began, form "the roughest part of the trail." Furthermore, she was often alone, having set out before the season began. That solitary presence and her age have made her an unusual sight on the A.T. Most long-distance hikers are either just in or out of college, or retired.

Some solo hikers use various safety devices. Cooley met an older Tennessee hiker who carried a loud beeper he could set off if he became injured. A woman Cooley saw on the last trip was accompanied by her golden retriever. "She had put a little pack on its back, so it could carry its own food.

"My personal opinion is that it's safe," she says. "This is just a different kind of risk than most people are used to taking. If something is important to you, you are willing to take risks."

With warm weather, Cooley has not only met hikers, but also found a "whole network of small Appalachian towns along the way. You meet people you wouldn't ordinarily. They are so friendly -- you get offered food, homes or backyards to stay in. They just couldn't do more for you."

She also enjoys meeting other through-hikers. "Often, they are hiking because they are at a crisis point in their lives, a time of change. That's why they have time to cover the trail."

Cooley has found that through-hikers are a casual cult on the trail, almost a family. They look out for each other at the shelters (wood-and-stone lean-tos about a day's hike apart all along the trail). At each shelter, notebooks are left by hikers for others to fill in. (When it's full, the person making the last entry is supposed to mail it to the originator.) Like CB-ers, through-hikers develop handles for themselves, with which they sign the notebooks. On her last trip, Cooley's habit of wrapping all her gear separately in plastic bags earned her the handle of "Bag Lady."

This trip, a hiker nicknamed "Milhunky" is leaving cartoons at each stop, instead of messages. "The last one showed a hiker with wavy lines coming from his feet. It said, 'Nose is supposed to smell, feet to run, not the other way around.' "

Trail life can be rigorous. Cooley has tried to keep a journal, but finds that in the evening a hiker's thoughts drift more toward food and sleep than Thoreau. Hot showers, she says, are something to dream about. "You have to enjoy being out there with the scenery and the wildlife," she says, "away from cities and large crowds."

Cooley likes the feeling of self-sufficiency on the trail, and the excitement of the trip. "It's a controlled adventure. You have maps, you have a destination, but what you get from your time on the trail is entirely up to you." ; ON YOUR OWN

You may not want to tackle the whole Appalachian Trail, but at this time of year you can bite off a good piece of it: The weather's warm enough to camp comfortably, but the air is not yet hot and sticky nor the foliage overgrown and thick with mosquitos. And with 492 miles of the Appalachian Trail in Virginia and 39 in Maryland, the weekend hiker seeking a primeval experience has a lot of choices.

Before you put on the hiking shoes and the two pairs of socks, says Cooley, be sure you've got a good, current map. Though the Appalachian Trail is marked with white blazes, it is easy to stray. Maps of parks are usually available at their entrance; the Potomac Appalachian Trail Club (PATC) sells maps for the A.T., as well as for numerous trails in the area. (See below for more information about the PATC.) Those planning a hike should first take realistic stock of their own experience and physical condition, elevation changes, the availability of parking, and water supply on the trail. (Water from lower elevation streams should be boiled before drinking.)

Cooley recommends a few favorite trails, some of which she trained on in preparation for her trip:

MARYLAND HEIGHTS -- "This is a nice weekend loop trail with an overnight stop." This trail in Harpers Ferry National Historical Park can be hiked in five- or 10-mile segments or an 18-mile loop. The five-mile trip is a fairly steep climb up the Grant Conway Trail to the ridge, passing several overlooks of the Potomac and Shenandoah rivers.

Cooley extends the hike at the top by following the Elk Ridge trail, a level hike to Buggy Rocks (this is the turning point for those hiking 10 miles), with a good view of the valley. From there she crosses the valley to overnight in Gathland State Park, then takes the A.T. south to the river.

"Don't miss Weaverton Cliffs before descending to the river, with a view of Harper's Ferry, the river and countryside." From Weaverton Cliffs, take the towpath along the river, follow towpath west back to bottom of the Maryland Heights Trail. Toilets, water, primitive camping area in park. (No car camping.)

To get there: Beltway to I-270; north to I-70; west to U.S. 340; follow U.S. 340 to sign for Sandy Hook Road. Turn right, drive three-tenths of a mile to a small parking area on the right. Follow the sign for the Grant Conway Trail.

WASHINGTON MONUMENT STATE PARK -- "It's a good place for car camping. There really is no circuit trip, so it's not good for long-distance hiking, but has a beautiful view." The A.T. runs right through this 100-acre park. This Washington Monument is a 34-foot memorial shaped like a milk bottle which crowns the park. The 150-year-old monument predates the obelisk downtown. Camping available along the A.T. or in the park, $3 per campsite. Toilets, hot water are available.

To get there: I-270 to Frederick, then I-70W. Exit I-70W at exit 49 (Alt. U.S. 40W); nine miles to the South Mountain Inn, right in front of the inn, follow signs to the park. Information: 301/432-8065.

CATOCTIN TRAIL -- Gambrill State Park. This trail approaches but is not part of the Appalachian. It is best to begin at either Cunningham Falls State Park or Catoctin Moutain Park, since overnight parking is available at both sites. Hikers may camp but not park overnight inside Gambrill State Park, which is 26.5 miles from Cunningham Falls.

Cooley made her own loop trail for this hike, with the aid of several maps. "I passed several still, perfect lakes," Cooley says. "I was on the trail for four days, and only met one hiker the whole time."

To get there: To reach Gambrill State Park (no overnight parking) from I-270, go right on U.S. 40 toward Hagerstown. The park is about six miles down the road. The Catoctin Trail begins at the nature trail sign. For overnight parking, take U.S. 15 to Route 77. Parking is available at the Owen Mills and Houck areas of Cunningham Falls State Park. Information: 301/473-8360.

SHENANDOAH NATIONAL PARK -- "This is a very good place to start," says Cooley, "with beautiful views." A booklet called "Circuit Hikes in Shenandoah National Park" is available from the PATC. Hikes range in length from four to 21 miles. Both Cooley and the PATC agree that the southern section of the park is less crowded. "Any trail that's not by a rest area has less traffic," says Bill Ladd, PATC finance chairman. One trail he recommends is Rockytop Ridge (Hike No. 17 in the booklet), a 14.7-mile circuit with a lot of elevation changes near Skyline Drive. The trail departs from and returns to the A.T. Information: see below.

MORE INFORMATION ON THE PATC -- The club (1718 N Street NW) publishes a variety of maps and guidebooks, and is open from 7 to 10 p.m. weekdays for walk-ins or phone calls at 638-5306. During the day, callers will reach a recording of information on the club's activities, which range from weekend trips to repair the trail to Friday evening "moon hikes" through the city. Hikers without experience may want to take a trip or two with the PATC before trying an overnight trip on their own.