So here this goes... I don't do it. I may be weird, but when I was single... I went on a streak of one night stands or booty calls. Though my current girlfriend has stamina issues, when apparently I last too long (average of 45 minutes to hour and a half).
Anyone who told you that girls want it for that long was misinformed. Your girlfriend is a trooper. It doesn't make you a good lover to go for an hour, really. It gets painful and uncomfortable to have some guy (and in your case, some fat guy) jack-rabbiting on their most sensitive part for that long. I can appreciate that you understand this and go down on your girl instead of constantly force her to endure you fucking her until her pussy is black and blue but patting yourself on your back for lasting is not really an accomplishment. Try to cum like 15 minutes in, I bet she'd appreciate that much more.
I doubt she'd ever tell you as much either. It's a sensitive topic to bring up.