With both of my kids I started walking when they were just a few days old (I think I waited a week with my second, maybe) but I definitely took it really easy and didn't push myself at all. After the Dr cleared me to exercise (4 weeks with my first, 6 with my second) I started doing a little more. At that point you can do whatever your body can handle, but you still need to be mindful of high impact exercise (running, etc.) Your bones and joints are still pretty soft from labor hormones and you can cause some permanent damage without too much effort (I know a lady who blew out both of her knees running a few weeks after birth.) They say to wait for 6 months before beginning a high impact workout routine, just to make sure your bones are hardened again.
"It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood; who strives valiantly..."