I avoid Falls Road. No shoulder and busy. There is a wide sidewalk on the north side of the road but I’m not a fan of that.
Seven Locks is my preferred route north. From Bradley to Falls (just near the I-270) is almost entirely well shouldered. The lower part doesn’t have a shoulder but there is a wide paved path to use or just ride in the road which isn’t very heavily traveled below River.
My route from this morning headed in for example (but in reverse):
http://www.strava.com/activities/133672401
If you are looking for miles, there’s other routes out in Potomac we can discuss.