The W&OD is easy to maintain speed on going west from Reston - you've got (busier) street crossings in Herndon, Sterling and Leesburg, but there are some nice stretches that get longer the further you go out.
If you've got the time and the legs, turn left at the end of the trail, then right on Main street in Purcellville to a Left on route 690. Turn Right, when you get to Snickersville Turnpike, then follow that out.
Once you pass the Bluemont country store, the grade goes (apparently) straight up - it's actually about a 11-15% climb for around 1/4 mile. The grade eases up as you approach route 7. Cross route 7 and turn left (careful of traffic!), and go up a 7% grade until the next intersection...then cross back over to route 601.
The first section of the climb up 601 is killer - grades approach 20%...but it smooths out to a 7-9% grade for the rest of the climb to the top of Mt. Weather. Turn around once the elevation peaks out at just under 1900 feet, come back down, and retrace back to Purcellville.
Going from my house was about 101 miles total RT (I live south of Reston). I had the bonus of a BIG thunderstorm hitting just as I turned around. It was fun descending the mountain without the benefit of brakes....
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