First off, congrats...there's no better experience in life than being a first time father.
Second, Well... just gave you the best advice you're going to get when he said that the best thing you can do in this phase of life is take care of the mother of your children. Go overboard. The support of your wife will be key during this period for your running.
Third, you're going to have your sleep disrupted when you have a newborn, so hopefully that marathon comes before the birth...or at least a couple weeks after. The first few days after the birth of a newborn - particularly your first - are a sleepless, nervous, exhausting blur. Your world will be completely turned upside down.
But...like another poster above, I had a great running season following the birth of our firstborn. It's not all bad! I learned to get a run in when I could, and ended up getting more quality wotkouts when I had to be focused.
Once things settle down (when baby starts to get into a routine) it will get better. Much better.
But I cannot emphasize enough...make sure your wife is happy, rested, and gets some of her own "me" time.