I learned to ride in Philly – you’ve prob got some of the best mountain biking within any US city in terms of the Wissahickon state park, about 5 or 6 miles out of centre city. Lovely ride out through Fairmont park (back of the art museum that Rocky ran up to) on the road; the wiss is then a steep sided valley that follows a creek.
It’s not massive, something like 5 miles or so, but there’s hard, technical riding there. Wooded, rocky singletrack – some of the climbs were brutal. There was a bit of access aggro when I was there (10 + years back), which is unlikely to have improved – just the usual tensions between a lot of user groups in an urban area. So you could check on what the score is there.
Another place we rode was Belmont plateau in Fairmont park – this was nowt special but did have local ‘races’ every weds in the spring/summer, off the cuff stuff where someone laid out a trail in the woods, great fun.
Further afield people rode in French creek, but only ever went once as I wasn’t driving at the time and it’s a stretch out of the city.