Patuxent Research Refuge is a nice run which is close by:http://www.fws.gov/northeast/patuxent/ntintro.htmlhttp://www.fws.gov/northeast/patuxent/FTP/NorthTractTrailsMapRevised08-25-2008.pdf
I second Wincopin and Patapsco:
Wincopin connecting into the savage trails is one of my personal favorites, within minutes you are in a rolling natural setting.
Patapsco between alberton rd (daniels) and McKeldin is a great 20+ mile trail run full of hills, muddy tracks, stream crossings and plenty of sights. The Avalon section also has a great trail.
If you are willing to drive there are some great trails in Potomac, Montogemery County, Carroll, and near Fredrick (catoctin mt.) all within about an hour drive from Howard county