Yes I said it, some of my friends go to strange new places that offer solitude that cannot be beaten! At your own risk of course. The parks I go to are swamped with people, dogs, screaming kids, and good folk that just make too much noise, or too many rules... Just within my county by asking around I discovered large places to hike and mountain bike that are very questionable, but are unlikely to be enforced. We have nearby abandoned quarries, and super fund sites that have been cleaned up and are covered in trails. Large old military bases that have been abandoned are just as interesting. West Chester University has a large piece undeveloped wooded property that mountain bikers have been using for a few years.
Here is one super fund site that I know about with loads of trails: