We have been having attention problems when lunging too and when he did not pay attention I made him go more. That did seem to help, but as you said you need a lot of time to do that of which I have little. I do lunge him when I can. I do not have a round pen so have to go down to the neighbors to use theirs. It gives him time away from his buddies which he hates and has a hard time concentrating at first but eventually settles and catches on.
Lunging is so boring for the lunger and the lungee:) I don't like doing it, but it does serve a useful purpose. But I think it can be over used and be counter productive.
I don't have a round pen. I'm not a big fan of them:), unless you use a lunge line in them.
Becky