It will take some weeks (4, 5 or 6?) for your uterus to pretty much shrink back down into your pelvis properly. Then things are pretty much down to weight gain (whether you have a CS or a natural birth). Your skin is likely to be pretty loose for a while (even if you loose all of the weight straight away - I lost all of mine within 2 weeks of birth, despite eating like a horse!). Depending on age, amount of weight gain, how healthy you are post-birth, you will probably be looking at the best part of a year to get completely back to normal.
Yes, you can do CERTAIN exercises straight away after a section (I was given a set of exercises to do the day after my son was born - by an amazing Osteopath who visited me in hospital).
I did a lot of pelvic tilts, pelvic floor exercises and "holding abs in" exercises - as recommended by the physio - during the first weeks after birth. They are brilliant. DEFINITELY avoid sit-ups - they can make abdominal seperation a LOT worse... You need to take it easy with your abs and build up your core muscles very gently - otherwise you risk throwing your back out....
I also started "walking" about a mile or two a day (gradually building up over the first week or two), with the pram, from two weeks after birth. Great for me and great for baby.
Then after my 6 week check, I started post-natal yoga (which you can take your baby to - make sure that you get a GOOD instructor and tell them about your section), swimming and the gym. However, before the birth, I did keep active: I swam 50 x 20m lengths twice a week (up until 4 days before my section) and did pre-natal yoga once a week.
Take it easy, give your body time: your body has just done the most amazing thing, so it needs looking after. Good nutritious food, lots of water and gentle exercise :)
QT