To start... There was a fun little area in Cole's Point (E of Fredericksburg and NE of Richmond). There is a marina/camping area there too (http://www.colespoint.com/home.html).
After driving off of Coles Point onto the dirt road, make a right towards a split in the road at the "T' instead of left towards the marina. There will be a wide dirt path to the right with a raised dirt area (canceled construction site), that alone was kind of fun but had lots of deep ruts so be careful. You will also fins the trail in this area that split a couple times each and some eventually lead back to other roads nearby. Larger SUVs might get their sides scraped up by vegetation and broken branches/tree trunks.

I was only here once for less than 30 minutes with my buddy navigating (and enjoying the ride and being impressed with the little Jeep that could). I do apologize for the vagueness. It is about a 2 hour drive south of Woodbridge/Springfield, probably half that for Fredericksburg folk

I used to live in Alexandria, but the only off road places I drove while there were at Ft Belvoir, or AP Hill, or other Army bases. This was while I was stationed at Ft Myers.

Cole's Point is a little over 2 hours south of Alexandria, but relatively easy to get to. Although from what I've seen, there is maybe an hour's worth of fun in the area I was guided through. I work on/near FT Belvoir and there were no trails open to public anymore. They are trying to "expand" that little base with new residential areas and whatnot. Someone I worked with there grew up in VA and was also stationed on the base for a while. He said they used to go off-road a lot on the base, but it was because they also had the keys to access one of the areas. :( It would be nice if there were some more public areas in NoVA.

Well, we were using Army Jeeps too, and in the military, so guess it was ok for us.

My daughter was born at the hospital at Ft Belvoir, over 30 years ago, guess I'm old, lol

Is the Fish Market still in Old Town? I used to like that place. Good chowder too.