If I don't exercise almost daily, my mood and sleep are terrible. There's no way I'm waiting until 6 weeks PP unless I physically can't do it. This sounds like an old-wives tale, related to cultural practices where the mother wasn't even supposed to step outside for 4 weeks PP. None other an authority than the Mayo Clinic says: "
If you exercised during pregnancy and had an uncomplicated vaginal delivery, it's generally safe to begin exercising within days of delivery — or as soon as you feel ready."
http://www.mayoclinic.com/health/exercise-after-pregnancy/MY00477