Audax rides

Mine was the Dean. As you are a lots of time for the pub rider I would strongly recommend the Hardboiled.

I can tell you what not to do: don't do your first 300 fixed, if you have a Garmin 800 don't record it all in one go and don't start your first one at midnight.

stretch every couple of hours, as soon as you get off the bike, I find it helps a lot.
start as early as humanly possible, you'll be glad of it in case anything goes wrong.
eat lots of carbs and fat and not too much sugar
glwr

Eat, eat, and, just when you can't face anymore, eat. Drink a bit when you eat too.

Don't use a Garmin for navigation.
Eat and drink often.
Enjoy.