I am not aware of any major trail systems in that area on the WV side, however hop across the river and head to Gambrills, or the Frederick Watershed in Frederick County MD (more information here
www.more-mtb.org - go to the "forums" tab, then "where to ride"), I know some people also ride at Greenbrier State Park (MD) as well.
I would like to know if there are any trails in the Martinsburg area as well as thats not too far away.....