I've been away from Philly for ~ 10 years now, but once you figure out the street navigation to the trailhead near Manayunk you can put on plenty of miles after heading to Valley Forge. See
http://www.schuylkillrivertrail.com/.
If you take a diversion before Manayunk, you go up the scenic Wissahickon valley which would get you pretty close to UPenn's botanical garden and some open roads. Can get hilly.