Walking is a great way to start down the road to fitness. Do something you like so you develop the habit of doing it regularly. You like to walk? Then by all means, get out there and put on the miles.
Weight lifting is a great way to add muscle, which will help burn
fat stores. It will give you a musclular butt and legs if you decide to incorporate it into your workouts. Your legs are your biggest muscle group and will add tremendously to increasing your metabolism when they become more muscular.
Lunges, squats, etc will all help to change the look of your backside.