AFTER 50 YEARS, the federal government has reversed its hand-off policy and pledged to protect the 2,000-mile Appalacian Trial.

The wold-famous trail, which follows mountain ridges from Maine to Georgia, was built and has been maintained almost entirely by volunteers. In recent years private landowners have barred hikers from hundreds of miles of the original path. Federal officials have deplored the trend while denying responsibility for reversing it.

"This administration accepts that responsibility," Assistant Interior Secretary Robert L. Herost told 900 cheering members of the Appalachian Trail Conference at Shepherdstown, W. Va., last weekend.

"It is my objective that we will be able to protect at least 300 miles of the trail's 600 unprotected miles now listed as the most threatened, by your next meeting in 1979," Herbst said. "You have my commitment (here he raised his arm like Billy Graham exhorting the faithful) that we will seek the funds - be they federal or matching - needed." He said Interior will seek $35 million for openers.

"It was an about-face if I never heard one," said Ed Garvey of the Potomac Appalachian Trail Club. The speech, delivered in stentorian tones that shook the room and started tears from some who had thought they'd never see the day, brought Herbst a standing ovation.

"The trail was conceived - and properly so - as a cooperative effort between the private citizen, the states and the federal government," he said. "In all candor, I would have to say that only the private citizen has come close to carrying a fair share of the load . . .

"I can't change the past, but I do have an obligation to direct future activities, and I assure you now that I feel that obligation very strongly." Herbst is assistant secretary for Fish and Wildlife and Parks, which puts the ball in his home court.

Herbst went to some lenghts to assure his listeners that he and the President are trails advocates. As Minnesota commissioner of natural resources he said he had overseen development of 7,500 miles of trails. He said he has adopted the Appalachian Trail as "a model for a national trails system."

Some 174 miles of the Appalachian Trail now run along public roas because so many private owners have posted their land that there is no reasonable way to rout it through natural areas. Another 662 miles are kept open by "cooperative agreements" that are not binding, hands, some stretches have been rerouted severa times in a single year, leading to a maze of old and new blazes that send backpakers wandering in circles.

Herbst said his department would take a flexible approach toward restoring the trail as closely as possible to an ideal route.

"Any agreement . . . must be based upon the equal partnerships of private citizens, the federal government, state and local governments and private landowers. Too often in the past we have assumed that national recreation resources can only be protected by federal ownership and maintenance.

Land uses such as farming and timber are compatible and exciting dimensions of a trail. The Appalachian . . . goes through farmlands, through timberlands, through communities small and large. It . . . builds upon all of these to provide an experience unique around the world."

Herbst said the authority granted by Congress (but never used by the executive branch) to acquire a 200-foot right-of-way "is not sufficient to protect the trail experience." He said he will consider recommending that Interior Secretary Cecil Andrus seek authorization of a corridor as much as 1,000 feet wide.

Herbst said he would send copies of his speech to all 50 governors and would specifically encourage officials of the 14 states through which the AT passes to take immediate steps to protect it. He announced matching grants of $140,000 to New York and $400,000 to New Jersey to protect AT segments, and said there would be more where that came from.

But, he said in an underlined passage of his text, "We are ready and willing to use the authority to condemn land should there be no action on the part of the states . . ."