Can sex last too long?

I'm trying to figure out if this is just me. Sometimes my friends will mention having "marathon sex" that lasts for an hour or more. Every time they do, I cringe. It's not that I don't enjoy sex while I'm having it, but even with lube I tend to get sore or the sensations down there stop feeling as intense. The problem is that generally, my boyfriend lasts a really long time. My limit is probably 30 minutes and then it just stops being fun. My mind starts to wander, I can't concentrate on the sex and it just stops feeling as good as it did when we started, whether I've orgasmed already or not. He is an attentive lover and I enjoy sex with him. But even with a lot of different positions and moves, eventually I kind of want to go do something else. I don't think it's a libido problem; I have a pretty high libido ... I just don't seem to have the same stamina as him. Am I just not getting into it enough? Is this something that other people have found with their sex lives too? Is there something I can do to make things better? I'm totally clueless.