Great Falls National Park and the Riverbend County park allow Mt. bikes on designated trails. You may start your ride at the main parking lot for either park or at the Difficult Run dirt parking lot located at Rt. 193 Georgetown Pike and Difficult Run (Approx. .5 mile east south east of the traffic light at the intersection of Old Dominion Drive and Rt. 193. Well its not Moab, but you can get a good workout in the wooded dirt road/single track combo. Great falls is really mostly road or non technical single track. River Bend located north and immediately adjacent to Great Falls has the better single track riding. There is an easy single track with great views along the potomac. Connect these two trail systems and you can get a good 1-1.5 hour workout just 25 min. from DC. (Bring your climbing gear for a ton of top roping in the park just south of the "great falls".
How to get there:
From DC take the George Washington Parkway to I-495 exit Rt. 193 Georgetown Pike (before actually getting on I-495) turn right onto Rt. 193 heading west towards Great Falls. Drive about ten minutes to first traffic light turn right to get to Great Falls Parking lot. This is the middle of the loop. You can head in either direction until the trails end (this is a good way to scope out the other parking areas.
Can be combined with Difficult Run Trails for longer (wetter) day.