Since you started out on the low side as far as weight, the pregnancy weight gain isn't as bad as you think. First, you need to listen to your doc as far as when you can get back to training and how hard. Especially if you wind up having a c-section (hopefully not) it will take even longer.
I used a post natal pilates tape. Here is the site you can get it from:
http://www.pilatesforpregnancy.com/
I loved it especially since it involves your baby also. Great for bonding. It also helps to develop your core strength and work all the muscles that are stretched out, but also takes into consideration that you just had a baby. As far as diet, your calorie and food intake is going to depend on whether or not you are breastfeeding. If you are, you need to make sure you are getting enough calories and proper calories. Eating every 2-3 hours will help your metabolism and also keep your sugars level, and help with the energy you are going to need. In the hospital ask about specifics as to what you need to consume if breastfeeding, and also you can ask your doc. And don't compare your body to pre-pregnancy. Your body is shaped different after being pregnant, and your hormones are not going to be any where near "normal" till at least 1 year later.
Good luck and best wishes for a safe labor and a healthy baby!