#1 rule is to NOT second guess yourself. You did the training. You did your race plan (!!). You KNOW that you can do this! Now, go do it for us! Good luck Rob! And hopefully David has an enjoyable trip too with a renewed motivation to conquer his Ironman next season.
One quick note that is more race specific. Since you are heading to Canada, remember that they do everything in the metric system. It’s up to you how you want to set your Garmin. A few change it to the metric system to match the course. Others keep it with the system you are comfortable with. Either way, you will likely be doing lots of math, which will help take your mind off of your pain and suffering. Or just thank the volunteers and emergency personnel for greater impact! 🙂 Good luck!