Last week, Marceline Guerrein retired eight years early from her secretarial job in Alexandria. Four days later she said goodbye to her husband and, with a one-way ticket in hand, boarded a plane for San Francisco.

Guerrein is coming home. But when she does, she will be walking.

With about 100 other hikers and backpackers, Guerrein is hoping to complete a 13-month hike across the country, sponsored by the American Hiking Society and the Interior Department's Heritage Conservation and Recreation Service.

The expedition is called Hike A Nation and is designed to draw attention to the national and local trail systems in the United States. Newspapers along the way are being urged by promoters to publicize the trek as the hikers approach, and the hikers are being asked to update maps and make suggestions to improve the trails.

Beginning at the Oakland Bay Bridge in San Francisco and ending at the Washington Monument in the District, the hikers will double-back on many of the roadways and trails blazed by settlers and frontiersmen pushing their way west.

The group will start across California on the Oregon Trail that will take them into the deserts of Nevada and Utah, across the Colorado Rockies and into Independence, Md. -- known as the jumping off point for the California-bound pioneers of the 1880s. The hikers will end their trip walking along the C&O Canal from Harpers Ferry and on to Georgetown.

Organizers of the hike say everyone in the United States is invited to join up and step out -- for a day, a week or the whole way.

Organizers estimate the hikers will cover about 5,000 miles. Each week they will hike for six days, take one day off and resume hiking. While daily distance will vary with the terrain, the organizers are hoping the hikers will cover an average of 12 miles a day.

There will be a fee of $1 a day charged to everyone in the group up to a maximum of 30 days. The money will be used to pay for a trailer to follow the hikers, and for a "hikers' hotline" that will enable the public to keep track the progress of the expedition. Members of the group are responsible for providing their own food, which they will purchase when they pass through towns along the way.

One of Guerrein's five children, 29-year-old Kurt, and 23-year-old Cindy Bain of Alexandria will be among those crossing the Oakland Bay Bridge this Saturday, when thousands of San Franciscans are expected to escort the band of hikers out of the city. Both young Guerrein and Bain have signed on for the entire trip.

Last week, Marceline Guerrein discussed her plans for the trip.

Dressed in a burgundy warm-up suit and sipping a sherry, Guerrein sat in her Alexandria living room just before her departure for California and confided that the decision to make the trip had been a difficult one.

"I didn't know for sure I'd be going until about a month ago," Guerrein said.

When when she decided, Guerrein said, one of the problems was telling people what she was planning. She is, after all, leaving a secure job and a warm house to sleep in a bag, walk all day and bathe infrequently.

"The people I work with are pretty much in awe, I guess. The only person who thinks I'm crazy is my boss," Guerrein said with a laugh.

Well, almost.

Carl Guerrein, Marceline's husband of 30 years, says describing his wife as "crazy" might be "putting it mildly."

"I'd be lying if I told you I'm happy about the idea," he said last week as his wife and Cindy Bain were busy making trip preparations. "But we raised our kids to be independent. It would be wrong for me not to want my wife to be that way, too.

"You could say this is kind of a dream of hers, something she feels she must do. I couldn't stand in the way of anyone's dream."

For Bain, who describes herself as "almost divorced," the trip is a chance to get away and think things out. A graduate of Fairfax County's Edison High School, Bain married young and took a job as a receptionist in a veterinary hospital. She says the hike marks a turning point in her life.

"I think I need a change of scenery," she said.

Asked what she fears most during the coming months -- bears, blisters, poison ivy or the like -- Bain shook her head and said: "Winter."

"I'm not afraid of snakes or anything like that," Bain said with a smile. "I don't care if I don't get a bath or my clothes are dirty. I just don't like winter."

According to the best estimates available, the hikers will be moving through Missouri, Arkansas, Illinois, Kentucky and Virginia during the winter months.

"At least we'll be out of the Rockies," Bain noted.

Bain and Marceline Guerrein were brought together a year ago by what Carl Guerrein calls "the hand of God" -- in the form of a note at an outdoors store seeking a hiking partner.

Bain posted the note and Guerrein responded. The two women have been hiking together ever since.

Guerrein does most of her hiking on weekends in the Shenandoah National Park and has never made a trip of these proportions before. Bain hiked from Georgia to Virginia on the Appalachian Trail for two months last year.

"It was a great experience," Bain recalled. "But I keep remembering winter and how hard it was to walk through all that snow."

Guerrein's son Kurt was persuaded to go after leaving the Navy earlier this year.

As the women discussed what they would be taking to occupy the time during the coming year when they would be tent-bound, Bain decided she wanted a tiny backgammon board and Guerrein said she wanted a small dictionary.

Suddenly Guerrein looked at the stack of official hike information in front of her and confessed: "I really have butterflies in my stomach.

"But, you know something, I look upon this as the greatest vacation of my life. It's something to tell my grandchildren about when I'm sitting in the rocking chair -- and I'm not that far away from it, either."