Here’s a route I just put together. Really, I think you need to decide how you’re going to cross 495 and 66. AFAIK, the main bike-friendly crossings for those (other than the W&OD) in the area are Oak St and Idylwood Rd for the former (but Oak’s a lot less stressful) and Virginia Ln or Barbour St for the latter (pretty similar IMO). I’m a big fan of Washington Blvd, and now most of it (but not all) has painted bike lanes. Other caveats with this route include the off-roading part in South Railroad Street Park (by the beginning) and the ramp onto Washington Blvd (over 66, by the in-progress W&OD bridge). I think you should be comfortable riding on well-trafficked streets, but it’s how I’d get from Dunn Loring to Rosslyn entirely avoiding trails.
Bentbike might also be able to help.