You have a pretty decent sex life, love him and want to swap that in for OLD? OP, you clearly have never been single in your fifties!
It sounds like your husband is a bit depressed and you are having an existential crisis. These two things can be tackled separately- does he need to see the GP or just to have this pointed out so he can find his own coping mechanisms (exercise, seeing friends, come out of it in time)?
You are having the same crisis as most of my women friends, and me, which is that we suddenly realise the clock is ticking and we would kind of like to be 20 and do silly things again and just live for the day.
Things that have helped me and my friends have been variously to recommit to work or change job/take a Masters/train to do the job you always wanted, this has been surprisingly successful for lots of us as many of us took our foot off the pedal during the childrearing years and there's quite a lot more still to do.
I would also try to reconnect with what you used to love when you were younger, your tastes will have changed, but if you loved 80's music, why not find a friend and go to an 80's night, or the cinema and then chat over dinner afterwards?
I find dressing with a 'hint' of my old self works for me, I've given myself a make-over in the past four years (since 50) and that includes better teeth, weight-loss and new clothes. It's not over yet unless you want it to be.
I agree with going out a couple of times with your husband and getting the conversation started about where you see the next decade going- this will tell you whether to book holidays for yourselves, even just cheap weekends the two of you in the UK if you can't do abroad, or set out alone to make your life more what you want it to be and hope he comes along for the ride.
Watch Shirley Valentine.