The A.T. through the mid-Atlantic was explicitly designed to be relatively near the major Eastern Seaboard cities. But you can drive out to the GET routes as quickly now as one could to the A.T. then.
And, though Duncannon may be a fine little borough, in equivalent position in PA along the GET is State College, a Big Ten university town with considerably more offerings such as an outfitter, Wegmans, Penn State ice cream, several brewpubs, etc.
Some answers were offered in this thread: https://www.whiteblaze.net/forum/sho...-Eastern-Trail