I'm in Fairfax and I usually go the The Cove Campgrounds in Gore, VA to hit the mud and ride the trails. I'm about to join NOVAJeepers and start riding with them. They usually ride GWNF, Crozet, Flagpole and a couple other places. Big Dogs is about to have their icebreaker event at the Cove next month, so you should check it out.