I tend to do more frequent but shorter periods of canvassing, where I would walk for hours in the past I limit this now to two hours, a good strategy for me is to schedule work till 4pm and then I canvass 4-6pm 2-3 times a week, it works well and even if I get zilch I'm safe in the knowledge that my contact details are now lying on their doormat and there will be some return on this, hope this helps
Graham