@miller67,
Are you not staying erect or are you climaxing too soon?
If you dont stay erect, deffinately try working out (you could have slightly high blood pressure), if thats not your problem, I like doing a lot of different positions switching it up makes it interesting from just having sex, to doing something fun.
If you climax too soon, thats not really something you can fix, that just means you got some good sugarwalls to dive in to, so just go more rounds , dont throw in the flag in the first just cause youre tired of boxing, take the time to really appreciate and enjoy the experience, youre talking about SEX, not some obligation at work.