Some guys don't get turned on as much as others by porn. There's no denying that, at any rate, porn pales in comparison to the real thing.
Are you currently on any form of medication? Sometimes certain meds can interfere with, ahem, bodily functions.
Also over-thinking about things can delay/avert orgasm. "Not being in the mood" happens to guys as well as girls!
But yeah as others have said, being able to go for a prolonged period of time will only make her enjoy herself (and you!) so much more.