Ok, something a little more useful. For the ride take spares as you would for a big day of riding: tube (even if tubeless, as there be rocks!), water (there is an opportunity to refill about half way round, but I tend to take enough for the whole day, typically 2+ litres), spare mech hanger, waterproof, (used mine last year to keep warm from the wind!), and take enough food to keep yourself sensibly fuelled, and the usual assortment of CO2 canisters, pump, tyre levers, spare quicklinks, multitool etc. It would be a long and hilly walk back if you’re not prepared!
Ooh, spare brake pads (I actually lent my spares to a rider half way round a couple of years ago – got them back in the evening)
Oh, and a first aid kit, and hope you don’t have to use it.
Might be overkill, but maybe a whistle.
And, yes, all the above (and more…) is what’s normally in my Camelbak for *any* ride… (now a running joke in the club that people don’t take stuff as I have spares of everything…)
And take a smile – its a great event and you’ll love it!