I started walking about 4 days after giving birth. DH and I went on a daily afternoon walk, about 1.5 miles out, and 1.5 miles back. We had to get out of the house!
Around 3 weeks pp, after the bleeding stopped, I started running, but slowly, and short distances - 2 miles a day.
At my 6 week checkup, after I got the go ahead, I added in strength training/gym time and upped my daily running to start training for a half marathon.