Core is another thing to strengthen after baby as it plays a huge role in everything we do, running included. A quick note on core, any movement you do that causes a budge in the middle of your stomach where the abs have separated (as mine have) is a bad core strengthener. I have to be very cautious with leg lifts from the floor and planks. Any movement you do, focus on contracting and pulling the abs into the back and keep the belly flat to strengthen the very important transverse abdominals.
Anyway, I started running about 4 weeks postpartum and I realize that doesn't sound like very long to most people, but I started a lot smaller. I was doing run/walk intervals for several weeks and didn't run 3 miles straight until 6.5 weeks and built up to a 10 mile long run (with 2 other shorter runs during the week) by 12 weeks. The lesson here was that even though I could maybe have gone out harder and longer and sooner as I have in the past, I wanted to be safe.
The last piece of education here I'll give is what my midwife of my 2nd child said. She said that the best time to heal and strengthen the pelvic floor is right after baby. The pelvic floor is responsible for preventing 'leaks' from urinary incontinence and holding the bladder and uterus in their proper positions and to not prolapse. I'm not a personal trainer but I've looked into resources for myself. Some things I've done are kegals, reverse crunches and hip bridges as well as general aerobic activity. Walking and running are great, gradual running. Doing too much too soon too fast won't help, it will probably in fact hurt you. If the pelvic floor isn't healed and strengthened well initially, it may not heal as completely later. I have had 4 kids, 3 of them very big kids (9.5-10lbs). I want to be active and competitive the rest of my life even if it's just with me, so I will take the time now to heal up well.