I've always been a big advocate of keeping it local, I only leaflet in a 3miles radius which has 25,000 homes.
but I only go 3 miles north if I go south I hit Hull which is mostly a crap area and the traffic is terrible to get anywhere, I can go 12miles north in the same time it takes me to go 2 miles south.
for some it might be better to work on a time rather than a distance, say only work 20mins from your home which if you live in the country might be 15miles or 3 miles in a town.