The Custis Trail ends in Falls Church at a "T" with the W&OD. The "T" is immediately after the Custis Trail takes a sharp left turn to go under I66. At the stop sign, a left will take you along 4 Mile Run, a right will take you ultimately to Purcellville. Following the W&OD west through Falls Church is a little difficult as it takes some roads and isn't well marked. But once through Falls Church, its pretty obvious.

You can look at the maps on the Friends of the W&OD web site. The Custis Trail meets the W&OD on this section near Patrick Henry Drive and I-66: http://www.wodfriends.org/map15.html