To give you an idea, here's my (incomplete) planning doc from the Mersey 24 in 2013.
Here's my instructions to my helper for the RTTC 12 in 2015.
Good docs, very generous of you to share.
I'll be riding it next year, aiming to better my previous distance - though I think that the main target with an event like this is just finishing, whether or not you've done it before! My club record is held by Mick Potts, so well out of my league (for now at least!).
One problem I'm yet to work out how to solve is how to ensure my Garmin battery lasts the distance as I have delusions of riding to power this time, though I expect, after 12 or so hours, that power will just be whatever I can manage!
Didn't manage to do a 12 last time as LEL left me too knackered, but I'm hoping to do one this time round to get the full series in. I think I saw in the CTT annual report that next year's national is in South Wales, though I have no idea how hard that would be to get in.