Lots of good info, to add my comments.
Two bottles, a must, I generally have one with a Carb drink and another with electrolytes. Food, you know what your stomach likes, mine doesn't like gels but I would take a couple with me, better to be able to finish with the help of a gel and have bad guts than not finish at all! A good breakfast is essential, and a banana just before you set off, that's the first hour or so taken care of, then another banana, then something to eat every 30 minutes or so (flapjack, apricots, figs, peanut butter sandwich, what ever you normally eat, but make it proper food, leave the sugary stuff for when you need it). Two tubes, two or three co2, a chain link, multi tool, tyre boot, instant patches (park tools), pump etc.. fit what you can in the saddle / top tube bag, leave the food in the jersey pockets, easy to reach on the move. I have been know to carry a spare folding tyre strapped to the bottom of the saddle bag on long sportive.
As for conditioning, get road miles in, turbo is good, but nothing beats road miles. I was fortunate and had 5 days in Majorca a week or so back, 5 days, minimum 50 miles per day, total 320 miles, plus it had mountains, not their steep short ones in Wales, but still useful. I'll be doing an 85 mile ride in a couple of weeks, although it's fairly flat, there is one 25% lump in it. It'll help me check if 36 x 29 is an easy enough year, if not I can go 34 x 29 if I purchase a new cassette for the 'winter' bike.
I like the idea of taking some pain killers with you, I've not done that before but is a great tip.
Other than that, you're not in a race, you'll be cycling in stunning surroundings, take them in, enjoy, cycle with others, talk to them, the miles will roll by and you wont notice them. Walking up steep hills gives you a chance to recover / rest, take that option if you need it, there is no shame, no one (or at least I) wont judge you
The ride itself is sponsored by Bianchi, anyone not riding a BIanchi needs to add some brake rub, me I'm going fully italian, Bianchi and Campagnolo, but then I am a cycling tart!