People spent a lot more time outdoors. Local yokels would have spent a lot of time with a bow; either hunting, or target shooting, or just showing off. Kind of like the rednecks today spend a lot of time with their guns. That said, warfare on foot was done with volley fire, in order to fire at the maximum possible range. Ya know, since that's what the other guy would be doing and in order to stay as far from hand-to-hand as possible. Mounted archers would have been the ones doing quick firing. They existed in many forms, from the chariot to the cossack to the camel jockeys. You have a point about walls though.