Do you want running water or are portables ok? Search both the travel forum and sports and fitness forums - you'll find a lot of reccomendations. Most local trails have at least port-o-pottys at the trail head.
Our loca favorites -
Accontink
http://www.fairfaxcounty.gov/parks/lake-accotink/
Bull Run Occoquan trail (start at Fountain Head Regional Park)
http://www.nvrpa.org/uploads/Images/content/bullrun/watertrailmap.gif
Billy Goat Trail has port-o-potties and flush toilets at different points
Great Falls NP on VA side has running water
Sugarloaf Mountain in MD