WHY?" SHE ASKED, was she celebrating her 44th birthday pedaling 50 miles in the rain, on an ungreased bicycle, struggling up Terre Hill, or Terror Hill as it was called by those in our group with breath enough to speak.

But what goes up must come down. The rain stopped. The squeaking wheel was finally greased. And the first day of our two-family bicycling weekend in the Pennsylvania Dutch country ended deliciously -- with a 12-dish supper cooked by a Mennonite farm family which opens its home to strangers -- followed by a peaceful night at a deserted youth hostel, housed in a 161-year-old country store.

Bicycling seems by far the best way of touring the gently rolling Lancaster County countryside. The hills, at least in retrospect, did not seem insurmountable. And on the scenic backroads we met Old Order Amish and Mennonites on bicycles almost as often as in their black horse-drawn buggies. The women and girls pedaled by sedately in long gingham dresses and white lace caps, the men and boys in pants with suspenders and black broad-brimmed hats.

The great advantage of bicycles, we found, is that we traveled slowly enough to exchange greetings with the "plain people" of Pennsylvania and to stop and enjoy their 19th-century countryside, as unchanged and full of horses, cows, covered bridges and one-room schools as a Grandma Moses painting.

American Youth Hostels (AYH) and other groups organize dozens of Lancaster County bike tours every summer and fall, usually two-day weekend trips that range from $38.50 to $99, including food, hostel or motel, an experienced bike leader and occasionally "sag wagons" for those who fall by the wayside.

But our group of four adults and two teen-age children decided to pedal off on our own with a 25-cent county map, purchased at the local country store, which shows covered bridges, and a copy of the Greater Washington Area Bicycle Atlas, a $3.50 AYH booklet with maps of back-road bike tours in the mid-Atlantic states.

The Atlas's Pennsylvania Dutch tour, which we rode on a Saturday, is a 42.5-mile circuit from the Bowmansville Youth Hostel down to Intercourse and back a different route. However, it took us almost 50 miles, because we got lost twice, and it helped us decide that Sunday would be a day of leisure -- on which we followed the county map for 18 miles in search of covered bridges.

Intercourse is a center for tourists and the Pennsylvania Dutch (Deutsch), who are almost all of German or Swiss descent, not Dutch. Its streets are lined with hitching posts and parking meters and almost every weekend there is a traffic jam of buggies, bicycles, cars and tour buses. There are chair and carriage makers, country stores filled with quilts and homespun, and the People's Place, a center of Amish and Mennonite arts and crafts.

For tourists, Intercourse also has motels and restaurants and a post office with a special mail box for those wishing a local postmark. (A number of America's racier magazines have used the postal facilities here and in neighboring Blue Ball and Paradise.)

How the town got its name, in 1814, is a matter of historical dispute. The two more popular theories are that it was an intersection of several country roads or courses, and the other that it was named after a local race course, the entrance sign reportedly proclaiming Enter Course.

The most memorable parts of our trip, however, were not the well-publicized spots but rather the back-road sights of farms and people, of barefoot children working in the fields, of horses pulling antique bailers and harvesters -- scenes that can be seen elsewhere only in the Smithsonian Institution.

An invitation to supper at one of the local farms has been offered for more than a dozen years to those staying at youth hostels near Lancaster. Non-hostelers are invited, also, if space is available at the kitchen table.

Phares Hurst is one of the outgoing Mennonite farmers who welcomes strangers to his 15-foot kitchen table and a tour of his farm. An Old Order or Team(horse and buggy)Mennonite, Hurst's ancestors were among the early 18th-century German followers of Anabaptist Menno Simons to settle outside Philadelphia.

"Our traditional lifestyle is typical of America 250 years ago. German is still our first language . . . and we still believe in simplicity, self-reliance and a lot of religious teaching," said Hurst. Most Old Order Amish and Mennonites disapprove of higher education, have never fought in wars, "and take no part in political elections," Hurst told two dozen people at his kitchen table the night we arrived. "But we do not resent paying taxes. It is a very simple thing to do in a country where you have freedom, especially religious freedom."

Sharing the huge meal with us was a bicylce tour group of American Women Outdoors, which organizes outings for women. They were staying at a nearby tourist home. There was plenty of food to share, including meatloaf, potatoes, tomatoes, noodles, corn, canteloupe, banana cream pie, shoofly pie, peaches and fresh dairy cream, bread and butter -- all homemade -- accompanied by pitchers of water from the farm's 18th-century stone-lined well.

Hurst's wife and some of his nine children -- "a medium-size family," said Hurst -- were seen fleetingly because he said they don't particularly approve of opening their house and lives to outsiders. Hurst serves the meal himself and welcomes all questions to help explain Mennonite ways to "you English."

After dinner he took the group, as he has taken groups several nights a week for eight years, on a walk through his chicken barn, where 6,000 leghorns commmented on the late-night visitors, and his 200-year-old dairy barn, where several of the bicyclists tried milking sleepy Holstein cows. The barn has no hex signs, he said, because "they are for tourists. We do not believe in decorations."

It was after 10 p.m. when the 53-year-old Hurst, who had been up before dawn, answered the last questions and escorted his dinner guests outside, each carrying fresh eggs for breakfast.

While money is not discussed, contributions are discreetly accepted by Hurst, with AYH suggesting at least $4 per person.

The Bowmansville Youth Hostel was locked and dark when we returned but the houseparents had left a back door open and we stumbled in and slept in bunks, women upstairs in a dormitory where the country store's calico and homespun once were kept and men downstairs beyond the 100-year-old pickle barrel and the communal kitchen. The kitchen still contains a platform where 19th-century children stood to try on shoes.

On Sunday morning we awoke to the sounds of horses trotting on the streets outside -- "church bells calling you to worship," as one of the hostel's historians has written.

Opened as a hostel in 1937, three years after the youth hostel movement was founded in this country, Bowmansville was one of numerous hostels along Pennsylvania's Horseshoe Trail, all with barns for touring riders to stable their horses. Tourists on horseback are rare today and many days go by without a bicyclist, said houseparent Roger Lawn, a computer programmer whose family for seven years has lived in a wing of the hostel and operated it "as a kind of part-time job."

After washing dishes, sweeping and cleaning the hostel and checking out (cost: $3.50 a night for AYH members, $4.50 for the uninitiated), we filled our water bottles and headed for an 18-mile ride, finding only one covered bridge but passing hundreds of sleek black buggies and handsome standard-bred trotters tethered to church fences or moving at a sprightly clip along the roads.

After some Lancaster County ice cream, a Pennsylvania German specialty like chocolate, we loaded our cars and turned toward Washington, three hours and 150 years away.

To arrange trips to Lancaster County call the Potomac Area Council of AYH for information or to join an organized tour (783-4943), Washington Women Outdoors (942-8677) or Lancaster Bicycle Touring (301-398-6609). Reservations to eat with Phares Hurst can be made by calling him at 215-445-6186.