If you're hitting the bag with the plan to prepare you for an opponent treat it as such. No opponent you're going to meet is going to let you land a 100 1-2's. Granted the heavy bag can be used for conditioning, but it's much more than that.

If you just want stamina you don't need a heavy bag, if you want to prepare for an actual fight you have to train in such a way. That means fighting as you would against a sharp opponent, being deliberate, working intently, moving away from their power hand while you are circling or retreating, setting up your shots, jabbing, changing the distance, working on specific things and situations.

Now working in rounds is great because for each round you can work on particular things, such as combinations, how to set your punches up, and how to move properly. If you can't do any that you're wasting your time, this is more important than the physical benefits of working the bag.