You might want to limit the tug play outside of work. Or only use the one tug he really loves while training obedience. That way he knows he's only going to get his number one reward while training and never outside of that.
I'm not sure what kind of exercises you're doing, but if its just heeling and simple retrieval work, that gets very boring, especially after 45 minutes. I know that if I get bored doing it, so will my dog.