Thanks. Comes from driving and biking around the area for over 25 years.

Sounds like you're going somewhere near Arlington Hall, the old military base.

USDS, eh? I've got a meeting at USDS tomorrow. Different office.

qmsdc15's route is probably the best way from Woodley Park to the River. From the Key Bridge and Rosslyn the Custis Trail west.

George Mason is not really a bike path. Officialy it is an on-street bike route, but there is a sidewalk available (not great shape). You can try it, but GM between Wilson Blvd and Arlington Blvd is too narrow and busy for my comfort level, with lots of parked cars lurking. Might be quickest way.

My choice would be stay on the Custis Trail to the end. When it crosses under I-66 and T's, go right and then left onto the W&OD. Just before you run out of woods and it crosses Columbia Pike, there is a little trail on the left that takes you into the neighborhood onto the end of 8th Rd.
If you can enter Arlington Hall on the George Mason side, go two blocks and left on Wakefield, right on 4th. That puts you almost at the GM side entrance.
If you need the Arlington Blvd side, take the first left onto Buchannan, turns into Pershing. Just before it ends take a left (might be 2nd) then right onto Pershing again. Puts you on the service road next to the Unitarian Church. Cross GM and you're there.

You can also get to 8th Rd from the Four Mile Run Trail side. From Key down Custis and W&OD would be about 8 miles. From Key using 4MR and W&OD adds about 6 miles, but it is flatter than the Custis.

If you need to go through Clarendon for any reason, or for a change of pace. From the Custis Trail where it passes behind the shopping center, cross Lee Highway and take Kirkwood. Switch to Jackson after Wilson Blvd, left on 6th and right on Irving. Irving takes you across Arlington Blvd. Then right onto the path and it's on the other side of Glebe Rd. More traffic and rougher roads, but not a bad alternate.

I know several other ways to get there if you need more.

added - Arlington has a good bike website with maps.
http://www.arlingtonva.us/Department...esBikemap.aspx

Wilson Blvd has an ok bike lane. Road is iffy in spot and lots of traffic, but is works. Take it to Irving to finish ride. Big climbs to get to it in Rosslyn or Courthouse, so personally I'd take stay on the Custis or Kirkwood.