Last night I finally got a chance to ride the Potomac Yard Trail in Alexandria (previously known as the Landbay K Trail) and it's wonderful. The whole thing runs alongside - or part of depending on how you want to think of it - a linear park from East Glebe to Braddock Road, you can see what it looked like last year here. It's pretty much the same now, but with more development across the street. As reported at the GGW link, the plan is to extend the trail north to Four Mile Run. If you haven't yet, you should check it out. The park should win awards.
The only issue I had was in going to the trail. Heading south on Crystal Drive, I got to the intersection of Crystal and 26th and I didn't know what to do next. Crystal Dr. becomes one way north at that point, and the only direction given to cyclist there are signs that read "Bike Lane Ends" on Crystal Drive and "Do Not Enter. No Bicycles" on the transitway [Admittedly, my plan when I started my ride was to ride the transitway because I thought bikes were allowed, so I was dismayed to find out they were not]. Back at 23rd, there is a sign directing people to the right to go to Alexandria, but I was looking for bike-specific wayfinding, with something pointing me to the Potomac Yard Trail or Four Mile Run Trail.