Depends on where you are, if you go to northwestern VA you can go to the Cove Campground in Gore for the big dogs offroad events ( http://www.bigdogsof....com/index.html ), as for local stuff, try to find a local offroading group that can point you in the right direction as far as local trails go.
edit: I forgot that Chesapeake is down south instead of in nova so that kinda rules out big dogs.