Capital Crescent Trail
This 11 mile trail goes from the Georgetown neighborhood of DC to Silver Spring, MD. It is built along an old line of the B&O Railroad, but is now paved. There are bridges and tunnels along the way as well as water fountains and places to eat not too far off the trail along the way. So if you are feeling ambitious try to tackle the whole trail, if not you can drive to several points along the trail and walk or jog at your leisure. Your dog will love the scenery and the changes in gradient presented from the tunnels and bridges.