Hey bro. You can have your wife simply give you a handjob so she does not have to exsert as much energy to please you. Sounds like she doesn't mind but it takes too much energy out of her to give u want u want. As long as you don't make it a demand from her.
This is a very real complaint and I have the same one. My wife also isn't interested in non-intercourse sex (i.e., oral, hand jobs, etc).
I suspect this is why a large number of men look outside of the marriage for sex.
Is there any age difference? Have you become more physically fit? What is her physical condition? Post menopausal?
No offense for good suggestions more information is needed.