The B&A Trail near BWI Airport runs clear down to Annapolis. Pretty much totally flat. ~20 miles one way I hear. I have only done 10 and back before. Very nice, shaded and flat, ~8 foot wide trail. It connects with the BWI trail, which circles the Baltimore Washington International airport. Nice easy ride. Runs thru wooded areas just parallel to many shops on other side of trees. Even the back side of a mall you will see. Easy access to food and water, bathrooms. The little ranger station is a cool sight. There is even a bike shop you pass the back side of near the 10 mile mark on the left. Here it is:

http://www.aacounty.org/RecParks/par...s/bandamap.cfm