I too have been surprised how flat my but has gotten! T have been walking about 3-4 miles every day for the last 2.5 months but no other exercise. So last week I started a beginner's HIIT workout to try and tone up and reclaim my butt. It's starts really easy and builds up with each workout. So far so good! Good luck to you.
The 30-Day High Intensity Interval Training Challenge
http://blog.myfitnesspal.com/the-30-...ing-challenge/