There is some of the best hiking in VA right outside Leesburg, including parts of the Appalachian Trail. The Piedmont Environmental Council offers great resources for hiking in Loudoun County:

www.pecva.org/anx/index.cfm/1,325,0,0,html/

The Banshee Reeks preserve is very close to Leesburg. Algonkian Park runs along the Potomac River just east of Leesburg, as does the C&O Canal (but on the Maryland side of the river, a short drive away up Route 15).

You are not too far from the Appalachian Trail (go out Route 7 to the west), or for something tamer there is always the WO&D Trail along an old railroad right of way that runs right through town and goes east or west. I agree that PECVA is a good resource. Even give them a call, the office is very friendly and helpful about anything concerning the local environment.

Appalachian trail!!! Go up rte seven west towards Winchester and you'll see parking on the left at the top of the mountain before berryville at Mt. weather. Or go right and trails are up that way in the retreat.

Harpers ferry in WV is beautiful and Maryland heights is easily a good 3 hours and as intense as you would like for it to be:0) there's a cool ap for iPhone to find trails and state parks too! Just run a search and take your pick. . . If u have a droid, I pity you until windows 8 comes out and crushes all that is Mac:0)

Red Rocks Wilderness Overlook is another good option for Leesburg

nvrpa.org/park/red_rock_widerness_overlook

Here's another good resource for hiking in the area...

http://www.midatlantichikes.com/