Taking a spin class will definitely help and when you do those walks, SQUEEZE your gluts.... Also, Squats.. with those three things, I think you will find improvement (at least I have)
As for the spin class, oh boy oh boy............. not only good for the butt (although, it does hurt at first ) but your legs will also shape up..
Spin class is my new favorite exercise.. I just love love love it!!!
Since having some foot issues, I had to switch from aerobics/Zumba to a no pressure foot exercise.. Tried spin and the first time I was hurting.. but after a few classes, I built stamina and now.. I simply can't wait to get back at during the weekday..
give it a try!! best of luck to you