If you are looking for a fun and challenging mountain bike ride that's not too long, these two trails are close by and a lot of fun.  Located in the George Washington National Forest, they will have you grinning the whole time (well, most of it, lol!).  For a longer ride, add the two together.
 
The Little Schloss Loop is 7 miles long.  You start on a gravel road, then turn off onto a steep fireroad for a bit over a mile.  The top traverse is quite technical and will likely require some walking.  But once the trail opens up for the final descent, it is a blast!  Details on MTB Project: Little Schloss Loop
 
The Cedar Creek trail is just under 6 miles, so about 11 as an out-and-back.  Basically, you end up riding along a rocky creek bed.  Not much elevation gain, but you still might find yourself winded from working your way through the rocks.  Details on MTB Project: Cedar Creek Trail.