There are several Items of interest to DC area cyclists in the Spring 2007 issue of Rails to Trails magazine.
There's an article on trails that cross state lines which features the Great Allegheny Passage. This 135 mile long trail - or group of trails - connects to the C&O on one end, and Pittsburgh on the other allowing for an (almost) all-trail connection between D.C. and Pittsburgh. Unfortunately the article isn't online yet. It talks about how originally there were just a bunch of little sections built by towns and counties, but in 1995 they rallied around one mission and eventually one name. It reminded me - on a much larger scale - of the "Prince George's Trolley Trail" (I'm calling it that until an official name is chosen). In addition they mention the Greenway Sojourn, an eight day supported ride from Pittsburgh to D.C.
There's a blurb about a GIS mapping project for the Grand History Loop, a network of trails connecting visitor attractions in D.C. Virginia, Maryland and Pennsylvania. Contact Heather Deutsch firstname.lastname@example.org to volunteer.
As someone pointed out to me, there's a stat about rails with trails - 119 such trails exist in the U.S making up 878.4 miles.
The "Destination" feature covers Delaware's Junction and Breakwater Trail. It's a really nice trail just north of Rehobeth Beach, but at 3.6 miles its not a day of riding by itself. You can rent bikes, there are plenty of other places nearby to pick up miles (including the start of the American Discovery Trail) and the area is easy to navigate with Delaware's great bike map.