It was a grueling day for 68-year-old Lady Bird Johnson.First there was the one-stop, three-hour plane trip from Washington to nearby Tennessee. Then came a 2 1/2-hour ride along narrow, winding roads to a political fund-raiser in this remote break in the mountains between Kentucky and Virginia, 425 miles southwest of Washington.

For more than three hours, she patiently signed autographs and posed for pictures. Finally, with only a handful of people still lingering in the dining room of the Willowbrook Country Club, Mrs. Johnson gave her Secret Service escort the high sign that she was ready to leave.

"Not yet, momma!" shouted her daughter, Lynda Robb, over the din of a five-piece country-rock band playing in the bar. "We don't let anybody out without shaking hands. And we've got to get all the sheriffs over here for a picture."

As Lynda explained later, "I campaigned for her husband, and now she's got to campaign for mine."

Lynda Robb's husband, Virginia Lt. Gov. Charles S. Robb, the 41-year-old former Marine aide in President Lyndon Baines Johnson's White House, is pulling out all the stops early, including the Johnson connection, in his race for governor against Republican Attorney General J. Marshall Coleman.

The late president's Great Society may be out of style on Capitol Hill, but here, deep in the coal-rich mountains of Appalachia, remnants of his programs are everywhere.

There are sewer projects, patches of four-lane highways, Vista volunteers, food stamps for unemployed miners and black lung benefits for disabled ones. Lady Bird, the dutiful mother-in-law, was dispatched this weekend to capitalize on the Johnson allure.

She didn't let them down. In a region where Robb already had raised $250,000, her appearances at two fund-raisers mined another $12,000.

"Each of us in life is here to be used," she said of the part she has been asked to play in the hard-fought Virginia race. "The trick is to make sure you are used wisely, and well. If you don't serve your children, well, we have a country expression, 'it's the best of our lights.'"

Her role in the Robb campaign, she added on the return flight to Washington, "is to support Lynda and ease her path.

"Politics is a very hard life. When Lynda and Luci were growing up, they gave me a lot of flak about being gone so much, campaigning with their father. They forgave me, although they used to hold it against us."

"It's probably better for all of us that mother traveled wth dad," said Lynda. "We saw more of him in the White House when he lived where he worked. So you see we have a reason for wanting Chuck to win. We'll be with him all the time in Richmond."

Lady Bird hopes most of her contribution will take the form of baby-sitting with her three granddaughters, at the Robbs' McLean home, and at the LBJ ranch in Texas after she returns from a trip to China in June.

But she will return to the campaign trail for a few days in each of August, September and October, "traveling with Lynda. I'm not a substantive speechmaker. That was Lyndon's role, not mine."

But the woman whom nearly everyone calls Lady Bird was the reason 100 couples from Dickenson, Buchanan and Wise counties paid $100 each here Friday night to attend a Robb fund-raiser that pushed the soon-to-be Democrat nominee's treasury over $600,000.

She didn't make a speech, but told the audience, "I've known Chuck longer than most of you -- 14 years -- since he was a tall, stern-looking marine, which he was for 10 years, and as a law student and son-in-law. I'm real proud to share him in public service in Virginia." Her brief remarks were greeted with prolonged and enthusiastic applause.

Try as she did to live up to her own advice that "I never heard a bad short speech," Lynda Robb seemed to relish talking about her duties as "a household executive and professional volunteer."

At one point, she grabbed the microphone from Bonsall Sykes, the Dickenson County Democratic chairman, and explained that "In our family we say, 'lose your breath, lose your turn.'" Having caught hers, she announced that 'Because we got here a little late, mother and I will be at the door to greet all of you."

At the door, the Johnson women kept their promise, listening patiently to stories about LBJ, and recollections of Mrs. Johnson's visit to nearby Grundy four years ago, when Robb was running for lieutenant governor.

One man, choked with emotion, told Lynda, "I saw your father when he flew into Fish Trap [across the line in Kentucky]. It was one of the highlights of my life."

Added a beaming Donna Leftwich, after getting Mrs. Johnson's autograph on a color photo of her 10-month-old son, "There's nothing like good bloodlines in an election. Look at the Kennedy's."

At a $50-a-person luncheon on Saturday at the Holston Hills Country Club in Marion, Pat Jennings, who retired two years ago as clerk of the U.S. House of Representatives, criticized Reagan administration efforts to curtail or eliminate Great Society programs.

"LBJ turned loose every program he had around here and the pipeline is still filled," Jennings said. "When Reagan was going to take food stamps from the welfare folks, it was okay, but now our farmers are beginning to realize those people bought their beef with those stamps."

Mrs. Johnson said she has "a policy of not talking about our successors." The nation's economy "has a history of hills and valleys, and we have to take into account current conditions," she said. But she predicted that "the best [of her husband's policies] will come back. It left indelible marks, such as civil rights legislation, and it wouldn't make sense to go back."

Later, discussing the difference in philosphy between her late husband and her son-in-law, Mrs. Johnson said, "Chuck is probably more conservative than Lyndon. He's certainly middle of the road. But that doesn't lessen his heart or hopes in any degree.

"They were both highly pragmatic individuals. Lyndon always thought that economic prosperity was absolutely essential to the achievement of his high hopes.

"Each was a product of his times," Mrs. Johnson mused.

The last stop on the 30-hour tour was Abingdon, where Mrs. Johnson recalled she has brought the cabinet wives in 1965 to tour the Barter Theater and stay at the historic Martha Washington Inn.

Hotel manager Ellison Ketchum escorted Mrs. Johnson to the newly recorated room in which she had stayed as first lady, startling honeymooners Chris and Candy Horner who watched in awed silence as the entourage paraded through.

Meanwhile, Lynda Robb was shaking every hand within reach, prompting Abindgon resident Margaret Craig to suggest that "you'd make some politician yourself."

"In the beginning," Mrs. Johnson recalled, "it was hard for Lynda. She is rather shy. But she's learning. It's a stretching experience."

But at one point during the weekend, as Lynda urged her mother onward, Mrs. Johnson shrugged. "My daughter, she's a terror," she said with an understanding smile.