I got your message on running your 1st marathon. Here are my thoughts. Your goal is to finish. To break it up, stop at each water stop and walk through. Do not drink at every stop, however. Drink when you need to drink.
Miles18-26 is the toughest. If you need to walk, walk. Do not go out too fast. Take your time at the start. These are some thoughts I'm curious to know which marathon you are running. Good luck.