I think you have the general idea now of where the trails intersect. Arlington county has some bike maps online: http://www.co.arlington.va.us/Depart...esBikemap.aspx. You can also get some maps at the Commuter Connections kiosks locally.

A good loop that I do sometimes is to take the Custis to the W&OD, take the W&OD back into the city, where it connects with the Mt. Vernon, then back to Rosslyn. It's about an 18 mile ride. The connection thru Shirlington is a bit hard to figure out, but after the first time you do it, it's pretty easy. The maps should help in that department.