boxing is pretty tiring and is a good workout, but you use pads, bags, bandages and crap to protect your hands. ever used a Muk Yang Jong (wooden man/wooden dummy) before? try punching and kicking one of them for a session and see how you feel after
they're used to condition parts of your body for full contact and they ****ing hurt like a bastard at first (think punching and kicking a tree at full strength repeatedly).
there's alot of pain involved in the training for wing chun, the conditioning you go through is hectic.
it always makes me laugh when you see people using training pads and stuff to hit - if you ever need to use your skills for real your not going to be hitting soft foam filled pads and there's not going to be much give in them to cushion the blow. so why train yourself and your body to expect that? part of training should also be the conditioning of yourself to be able to take the effects of giving and recieving a hit - not just learning the motions.