You're welcome! Arlington County has an excellent bicycle route map on line. Fairfax County's map/system isn't as good but it does exist - check the FABB site for info. If the trails are clear, 4 Mile Run to W&OD to Gallows, then north on the Gallows Rd side path (I would try to avoid riding the roadway itself dues to traffic volume, speed, and driver ineptitude!) will get you into the Tysons area. Various routes once you are in the area depending on where you need to go (a side road crosses Rt 7 at Kidwell and another at Westpark Drive crosses Chain Bridge/Rt 123 if you're in the offices in the NW quadrant of Tysons). Or multi-mode and use one of the buses on Gallows Rd to avoid the heaviest traffic. Westmorland St and Great Falls Rd work better for people heading into the northern section closer to McLean. Rt 7, 123, 495, and Dulles Toll Rd/267 form choke points & bottlenecks that have been difficult to cross on a bike or walking. The current construction (Beltway widening and HOT lanes, Metro extension) has made it worse; that should clear up in about a year or so. You can see some of it on Google Maps/Earth.

Like ArtM said - after a snow/ice storm & cold snap, the snow is quickly packed down, rutted, and freezes solid. Because of shade and the orientation of the trail, icy spots can linger for weeks! Not fun to ride on and very hazardous on skinny tires (don't ask)! 700x35mm studded tires have worked OK for me although I found plowed/treated side streets a better alternative out in the Herndon/Reston area. A friend with more gumption (also a better bike handler and heals quicker than I do!) had good results with wider MTB tires and low pressure. We sometimes get extremly slick black ice conditions (more than my studded tires could handle) although those conditions usually don't last long. We're far enough north to get serious winter weather and far enough south to get more ice and lots of denial that a season called winter exists! OTOH, you will have a much longer riding season than you had in Boston - I started a new commute to Chantilly in March and checking my log, I did not see any weather-related drive days in March 2011.