Surprised myself
I got the totally random urge to take a sick day and see how long it would take me to bike to work.
I got from Springfield VA to Bethesda MD (19.5 miles) in 1 hour 45 minutes. I'm 29, could stand to lose 20 or 30 pounds, and don't exercise. The only significant activity I've had in the past 2 years has been a recent move, and some skiing in the winter. I expected to be dead when I got to work or perhaps somewhere on the way back but I made it all the way. The trip back took half an hour longer between walking some hills, waiting at more intersections, and taking a few breaks.
The ride itself is decent. Fair amount of wide roads or bike lanes and on the north side of the Potomac the Capital Crescent trail climbs a very slight grade all the way to downtown Bethesda. This could be a really good way to get in better shape and wouldn't be quite as long once I can go faster. Still it's going to take a lot of motivation to do this once or twice a week.
PS: Driving takes 40-45 minutes with the usual backup spots. Bike takes 1:45. Riding the bus and train.... also 1:45. There's a reason people still drive.