Personally, I think it would be best to shoot for a year after, not 6 months. I gained tons, had a c-section, and managed to Boston qualify when my son was 13 months old. I did a half-marathon when he was 5.5 months old - though I was slow then. (Pushing the stroller during the race didn't help!) Also, I was breastfeeding - and still am - and that can add some challenges. Take the time to enjoy your baby. Return to recreational running at around 6 weeks if the dr. says its okay. Set the marathon goal after baby is here, after things have settled down and you see how your body responds.