There is a 45+ mile trail that goes from Purceville, VA to Arlington. I was out there this past week visiting my sister and it is AMAZING!!! I would move there just so I could do all my running on this trail. It is all paved, very wide, no cars allowed, tons of runners, tons of bikers, tons of walkers, some mild hills, tons of trees, some shady areas, some sunshine... PERFECTION!!!!
I only ran 10 miles of it. WOW! I wanted to run for days. It was amazing. Peaceful and exhilarating all at the same time.
Bring your cell phone with you just in case you get carried away and run a few miles past where you parked your car and then get lost like I did. :)