You are partly right. When you trained a lot on 50 yards, 20 yards LOOKS rediculousy easy. However, if you havn't practiced this distance some time and only 50 yards, you'll shoot all your arrows plain over the target.
Therefore I practice all distances during a training-session. I have really fun in shooting one arrow at 10 yards, second on 20, thrd on 30, 4th on 40, and so on. I repeat this many times. When there is a distance I am not secure with, I practice that a bit sole.
In this way you keep in touch with all the different distances...., for each distance has its own challenge....