Walking on an incline will have a bigger impact, or so I understand.
I run rather than walk (and my knees are feeling it!) but in the past 4 months of running my butt has seriously changed! However, I went from not having a butt (got picked on for it all the time!) and I now have a nicely shaped bum, I like it! So there was some serious toning up that happened with my running.
Don't know if that will help at all, but I just wanted to say that with time you will see results if you stick with it and push yourself!