I’m one of the riders that did it originally, my tips.
You will need to get a couple of 7-8 hour rides under your belt, not so much for the fitness as I’m sure you’ll get that nailed no problems, but to get your bike set up spot on as trust me you’ll suffer at some point and you don’t want anything magnifying the pains you will have. They will also help you suss out how often and what to have food and drink wise.
Then even on day 1 use plenty of chamois cream, get the pad in your shorts full of it before you then apply some to your skin too. Otherwise the shorts just act like a blotting pad and soak it up.
Any more questions give me a shout (email in profile)