Other than light walking...you really need to NOT be doing anything strenuous until your doctor's visit around the 6 week point.
I have had two LARGE babies (10lb4oz, and 9lb4oz PREMATURE, respectively) and there are many reasons for the "pouch".
There are many reasons to take it easy. Pregnancy is much more than just the belly getting larger with the baby. Your body went through many, many changes to accomodate that growing child. Your internal organs actually SHIFT positions in your body cavity as the child grows!!! Your heart and lungs are included! Not only that, but your body produces more blood volume, different hormones, your hips "loosen" from their joints...many, many things happen during pregnancy. It takes the 6 week period, not only for the uterus to slowly shrink back down, but for all of the other things that happened to return to normal.
The "pouch" right after having a baby is from multiple things-your uterus went from the size of a lime to large enough to carry your child, the placenta, the bag of waters...it needs time to slowly shrink back down, and for it to "empty" of the tissues and blood and such. (Nursing will help speed this process-sorry if I am being too descriptive. LOL) Also-when you are pregnant, your core muscles weaken. I had a lot of back pain after my children, simply because my back muscles and abs were completely weakened from carrying those large babies. It took a few months of Pilates and ab and back exercises and dancing to get things back where they should be-it helped the appearance of the pouch some, but more than that-it helped me feel better to strengthen them again (no pain) and regain my proper posture.
The third reason for the pouch...your muscles are not only weakened in this area...but so was your skin. Most women who have small babies of 5 or 6 pounds don't have too much trouble in this area...but regular to large sized babies give stretch marks, and the skin doesn't "bounce back" always. I have really, really worked my abs and back...and lost weight...but still, after carrying around a 10lb+ baby...the skin in my lower abs will never be taut and firm again. It just won't, and I have accepted that. I do what I can, use firming lotions after I shower-and it helps with the appearance, but it just won't go back to the way it was "pre-baby". Motherhood does change some things for good...but I can live with that.
My children are worth it...and besides, my lower abs where the problem is is covered by my clothes, and noone sees that besides physicians and my husband, so it doesn't really bother me.