I know your ride pretty well -- I use the W & OD from Vienna to Falls Church a couple of times a week, generally, and I ride out to Reston on Sunday mornings when I get the chance. MY B-i-L rides the opposite direction -- Herndon to Merrifield -- and he finds that cycling takes about the same time as the car, but as you are effectively reverse commuting , that might not be the case for you. I know he does the ride from Herndon to Merrifield in under an hour, and depending on your capability, I'd think you'd be close.

Echoing Kegoguinness, I'd guess you can average over 12 MPH, if not right away, pretty quickly (depending on your ride, etc...) I average close to 15MPH on the stretch between FC and Vienna, and my average is only lower (a hair under 13 overall) because of the lights at rt. 7 and Gallows, and my street riding in the Town of Vienna after I leave the trail. The part of the trail from 123 to Reston is much faster overall, I think -- I average well over 15 MPH there. I ride a not-terribly-fast Kona Dew Plus (42c tires?), with a fair amount of commuting stuff, and I'm over 40, and still 25 or more lbs overweight.