Ballston, VA to Rockville, MD on the Rock Creek Park trail
Take Custis Trail East from Marymount University in Ballston. At Rosslyn cross the Key Bridge to get to M Street in Georgetown. Head East on M Street until you come to Pennsylvania Avenue. Just around the Four Seasons hotel turn right to join the Rockcreek park trail. Head North until you come to the end of the Trail about 20 miles later.