Re: practicing defense alone
You should be practicing defense all of the time; you should never just stand there and throw punches at a bag. That is how you teach yourself to be stupid in the ring, to just throw punches and then wait to get countered. Everything you do should start with a defensive move, so before you hit the heavy bag, you slip inside the jab with a right to the body, bob under and weave to the outside with a left hook to the body, and, once outside finish with a right hand over the top, and a left hook to get you right again.
You start preparing yourself mentally, on the bag, for what you will encounter against a live opponent.