Green Ridge State Forest - Mtn. Bike Trail features 12 miles of hiking trails near Flintstone, MD.
West on I-70 past Hagerstown to I-68 west. Take exit 64 and follow the signs to forest headquarters. Trail is approximately 2 - 2.5 hours from Baltimore, MD.
http://www.tripleblaze.com/trail.php?c=3&i=1737