I've given birth to three children and getting back after birth has not been the hardest part for me. It's when I got back to working full time combined with sleep deprivation that has made me pause my running for a while. But everything sorts itself out eventually. :)
That part is hard too! I found that to more mentally/emotionally taxing then physically taxing though. I still feel sad that I don't get the snuggles all day long when I'm at work. Everything does eventually even itself out! My six year old is very independent and doesn't need me very much at all anymore. In fact, she doesn't really like it when I pick her up from after school care. She wants to keep playing with her friends rather than go home with me!