Your ass is perfectly beautiful, feminine, round... spankable!
Honestly, women with a body like yours should never worry that their ass may get "fat". That simply won't happen. What you see in the mirror is exactly what makes you so delicious looking: that round shape that wants to be squeezed, massaged, caressed, and felt all around.
So forget the diet, but I like the idea of your riding exercises. If it's the kinda riding I'm thinking of, I can't wait till you post some sexy pics of it here! And I agree: nothing beats sex for a great work-out, exercising all muscle areas of the whole body.