MANY OF THE FOUR million hikers, naturalists and adventures who take to the Appalachian Trail every year are now enjoying its delights even more than usual. Autumn's show of colorbursts is an almost irresistible lure along the 2,000 miles of footpath that wind through 14 states from Mt. Katahdin, Me. to Springer Mountain, Ga. Whether it consumes an afternoon, a day or the four months needed to hike the full 2,000 miles, the experience of the trial offers much of what Benton MacKaye, the pioneer regional planner, first saw in its splendors in 1921 when he outlined his plans for this natural resource.

One of the misconceptions about the trail, however, is that it takes care of itself. It doesn't. A few days ago, several witnesses testified before the House subcommittee on national parks and insular affairs and documented some of the ways in which the trail is now endangered. Mainly, it is the pressure of development, pressure that is both severe and increasing. Because the trail passes within 100 miles of New York City, Philadelphia, Baltimore and Washington, and is within a half day's drive of 150 million citizens - 60 per cent of our population - its charm and beauty offer tempting sites for real-estate developments, trailer courts and other encroachments from the urban world. It is true, and perhaps miraculous, that no fast-food outlets have sprung up along the footpaths. But at least 800 miles of the trail currently have no protection. Already, 180 miles follow roads because right-of-ways have been blocked. As one of only two national scenic trails - the other is on the Pacific Coast - the Appalachian Trail comes under the National Trail System Act of 1968. But as Sen. Charles Mathias (R-Md.) and Rep. Goodloe Byron (D-Md.) and a coalition of conservationist groups pointed out, the law has proven inadequate.

In offering corrective legistlation - with the support of the Interior Department - Sen. Mathias and Rep. Byron seek to increase the width of the trail from 25 acres in any one-mile stretch to 125 acres. The proposed authorization in the Byron bill for right-of-way acquisition is $35 million. The funds could be used for direct acquisition of land by federal grants and by matching grants to the states. According to Assistant Secretary Robert L. Herbst, this would provide enough funds to protect 300 miles of trail.

These concerns about this natural treasure are well placed, and it would be unfortunate to let this opportunity pass. A decade from now we might awaken to discover that the integrity of the trail has been irrevocably violated. A little forethought would, like the trail itself, go a long way.