Pretty much exactly what Jared said and I agree definitely plan on hitting trexler [near allentown] sometime, that place is awesome. Wiss & Brandywine are both good places but brandywine has a bit more mileage. Below is a list of trails within an hour of media, each with its own unique qualities. Please let me know if you need any info on where to park etc.
I live near Marsh Creek in Downingtown so I frequent the following trails.
Marsh Creek - There are 2 sides the dam side and boat launch side both are good. Not too challenging but fun.
Harmony Hill - Downingtown [downhill or uphill]
Saw Mill Trails - Downingtown [lot's of climbing and off camber roots, a bit challenging]
Betzwood - Valley Forge [can be muddy, worth going to]
Belmont Park - Philly [more log overs per mile than tree farm after a tornado passing]
Wiss - Philly [long climbs, some fun downhill, some cool drops and a bit technical]
Brandywine - PA/DE on route 202 [some technical areas some good descents can have a lot of milage]
Fair Hills - Maryland [great system similar to white clay]
White Clay / Middle Run / Judge Morris (all interconnected) - Newark DE [extremely flowy, dont ride these trails during or shortly after a rain - lots of mileage, not technical at all. Lots of great rollers, well worth the trip]
French Creek - Rocks, then more rocks, then climbing with a side of rocks.
Philadelphia Pump Track - Currently under construction opening in May.
Nockamixon - Awesome flowy trails
Launch Bike Park - Spring Mountain - downhill
Places that I havent been to but are close:
Swarthmore College Trails / Crum Wood / Smedley Park (behind Springfiled Mall)
St. Petes - near french creek, west of philly by about an hour
Bonner Park - Philly (glen mills)
John Heinz National Wildlife Reserve - Philly
Ridley Creek State Park - Media
Core Creek - North East Philly (Langhorne)
Neshaminy: North East Philly