The Regent's Canal towpath is a wonderful, and beautiful ride...but often very crowded with pedestrians as well as being narrow and muddy in parts when it has rained.
There is a segregated cycle path that follows much of the lower (nearer the Thames) sections of the Regent's Canal - which can be found slightly away from the canal, up the hill. There are signs pointing it out. Can be a good alternate on busy pedestrian days.