Like everyone else said, take one of the Arlington bike routes down to the Custis or W&OD Trails and head west. The Arlington County/Falls Church boundary is about mile 6. Reston Town Center is at mile 18, you can get off the trail just before crossing under Reston Parkway or there is another path up to Town Center oppossite the ice rink/pavillion (entrance to this path is towards Sallie Mae and the Town Center Parkway overpass). Grade is pretty gentle; there is a long down grade from Vienna that'll slow you down on the way home; a very short steep climb just west of the Hunter Mill Rd crossing, and a general uphill grade to Reston Parkway. Several of the road crossings get busy in the mornings - Gallows Rd, Maple St in Vienna, Hunter Mill, Sunrise Valley Dr, Sunset Hills Rd, Wiehle Ave in Reston. You can check a map of the trail at the Friends of the W&OD site: http://www.wodfriends.org/map1.html and there is info at the bikewashington site: http://www.bikewashington.org/trails/wad/wad.htm

BTW, May 20th is Bike to Work Day and there is a morning event at the pavillion at Reston Town Center, you can get a t-shirt, quick breakfast, and there may be a drawing for some goodies. Check the bikewashington home page or waba.org

If you don't feel like riding that far, bike to West Falls Church Metro and take one of the Fairfax Connector buses to Reston Town Center (it's becoming a major hub for bus routes). Or get a folding bike and carry it on the bus (unlike Arlington County and Metrobus, the Connector buses DO NOT have front-mount bike racks).

Do not even consider biking down Route 7. I've crossed it during bike rides and I felt like I was putting my life at risk. Even with a traffic light.