I really like the trails around Fountainhead Park.
We also go to Manassas Battlefield regularly.
Sky Meadows is pretty and his a nice mountain part if you want to give the horses a good workout.
The Arboretum or James Long Park are okay just to get off-property for a little while. One thing I like about the Arboretum is that there is a wide variety of use - so it gets my boy used to dogs, bicycles, hikers, cars, etc.
Nokesville Horse Society keeps up a good list of trails NHS Trail Directory
Shenandoah National Park has horse trails, although I've never been on any of those (a little far from me, especially since I'm begging others for rides in their trailers).