The Pennypack Trail starts at the end of Pennypack Road in Huntingdon Valley, PA. Continue straight till you join up with the main gravel trail and turn right. Follow the main trial all the way across the city of Philadelphia to the Delaware River. The very last portion is fenced off because of the Correctional Facility, so you can't actually get to the river on the trail. Turn around and complete the run back where you started.
There are only a few roads to cross which makes this quite unique to be able to run all the way across the city of Philadelphia virtually un-interrupted.