I was surprised this park wasn't mentioned already.
The park has trails, trails, and more trails. The one I ran most recently actually required fording a stream. There are both pathed and single track trails to use.
The park is spred out over a large geographical area and there are plenty of free spots to park. I was there during the week and it was fairly uncrowded. I don't know about weekends. If you're visiting the DC area, it's well worth a run there.