I love to ride in Prince William Forrest Park. It's a National Park and very well maintained. They have a "scenic route" in the park that is paved and on the back half motor vehicle traffic is only one way. The other lane is marked off for cyclists, joggers and walkers. The back half is also relatively flat and only 3.3 miles. The back half is pretty hilly and a couple are monsters but they are good to test my progress. I still usually have to stop and walk or rest a little.
There are lots and lots of road bikers out there training too. They are so inspirational to me!
For mountain bikers the off-road is great! There are probably 8 fire roads and the map you're given at the entrance to the park gives you an idea of the difficulty of each one. My husband LOVES the Moave Road. It's long and hilly and very rocky.
So there's something for everyone at Prince William Forrest Park. If you're ever in the Northern Virginia area, IT'S A MUST RIDE!