Having had the experience twice, I will tell you two things that I think are super important:
Keep it simple.
Don't think about the whole thing.
I know you guys are super nervous, but you'll be amazed at the sense of calm that'll come over you as you bob in the water waiting for the canon to go off. When it does — there won't be any room for nerves or self doubt. You'll just get on task and proceed to make your way through the day. You'll handle whatever comes your way and you'll eventually cross the finish line. Much like giving birth, you'll forget whatever pain you experienced along the way. I know guys — you don't get it, but you will. The next day, what you'll remember is how awesome all the volunteers out on the course were, how great it was to see your family cheering for you in the chute as you came out of the water or how glad you were to see them when you finished that first loop on the bike or the run. You'll remember the spectator who said to you to keep going cuz you're gonna be an Ironman today. You'll remember entering the oval for that final lap and high-fiving complete strangers who are crazy happy to see you. And, if you're lucky - you'll actually hear Mike Reilly say your name :) ENJOY IT!! The day goes by in the blink of an eye.