I have never ventured as far out as 4 mile trail and Custis. Worth the trip?
Going West on the Custis along 66 is great if hill climbing is your thing. Once Custis ends at Four Mile Run/W&OD (can't remember which it is there, but it's all the same trail) you have a gentle downhill coast all the way to Shirlington. Here, you have to cut through Shirlington, stop by Cap City Brewpub to refresh, then climb up and over the 395 bike trail overpass and down into Parkfairfax. A 1/4mile along a street reconnects you to Four Mile Run trail on the other side. That last part of the trail is grungy - along the creek which is paved and underneath a series of bridges - Route 1, railroad, GW Parkway as you suddenly pop up on the Mount Vernon Trail.
I don't know if I would do it for the scenery overall. But the Custis trail's hills are a worthy adversary. You can do it in reverse and the hills are much less difficult.
