If you don't want to go so far, there's the St. Anthony Wilderness - Stony Creek Rail Trail, that's north of Harrisburg, PA. The A.T. runs parrallel and then bisects it. Camping is only at Rausch Gap shelter. The Rail trail is 20 miles long. If you start at Goldmine Rd. parking lot its only 3.5 miles to Rausch Gap shelter. I did that on July 1 & 2. The side benefit is just like the Greenbrier River Trail - swimming in the creek!