Some very good advice in this thread. Proud of you B&B.

I would say pretty much what SusanRachel said.

Most first time marathon runners never raced anything longer than 1/2, so the pace could feel slow and there is a great danger in that.

I don't think you have enough endurance to run what the pace calculator would suggest based on your best half.

Marathons have two halves which are not an equal distance. Stay on 4:20-4:25 pace till the 20 mile mark and than the second half starts.

Have a good experience. There will be a plenty of time to run more aggressively with the more training if you desire to in the future. 

I really like this advice. For a first-time marathon, I was always told to run it to finish. You don't want to run it too fast and than blow up at the end. I can't offer much advice myself because I've only run HM's myself, but will be doing my first this April. I can't wait to hear all about it.