For women, we have hard time with the lower abdomen region. When you do your cardio.. Do an interval cardio setting 2-3x a week. That will help a lot with the muscles, fat, and skin in that region. It's hard but start off doing 10 mins and working your way up.
Elliptical and precor are great for it because you can raise the resistance. Treadmill raise the incline. Elipitcal takes alot of pressure off the knees and ankles, so that may help you.
I had 4 abdominal surgeries in 3 years due to renal cancer. So I feel your pain! You will be surprised how the muscles will work back. Just takes dedication and time.