Annapolis here! I'm still on the search myself. I'm probably heading out to shenandoah again next fall and try to find some trail riding. I bet there has got to be some stuff on the eastern shore, the only little trails that I have found are on peoples property, and they are VERY short.
I've also heard of asseteek (however it is spelled, I just moved up here) island and you can get a pass to drive 4wd on the beach and camp anywhere on the 20mi stretch. Was thinking about doing that with some friends this summer. A little bonfire, surf, 4wd, and beer could be fun.