Trail running is my new sport.... I have ben going to Clayton park over off exit 11 on 195, I can get you better directions if your interested - good mix of hills and a really pretty place. The loop I do is around 5 miles but you can slice it up any way you want for more or less miles
Also mercer county park if pretty fun, but flat. If you feel like going to the shore Hartshorne Huber woods is awesome and Phila. has Wissahickon.
Trail running is fun, but I keep wishing I was on my MTB!