My first ever bike event was a 12hr solo event. Blissful ignorance might’ve been key, i only crashed on my last lap too 
i just had lots of different types of food and drink in my car (close to the “pits”), and a change of clothes + clothes for every eventuality.
I was there on my own so definitely made it up as i went along.
TR Gran Fondo Plan, and slowly work volume and time on the bike up as much as possible leading up towards the event. Probably a Masters Plan so you can add in additional volume if you have the time.
Priority should be volume and time on the bike as you get closer if it’s more about having fun, and you’ll want to make sure you fuel some of your long rides like you would during the event too.
My end of season race (December, I’m southern hemisphere) is a 401km MTB race and throughout the year I race XCM. I use plan builder mid volume configured with one XCM A race mid-year and then my other A race is the 401km one. I then have about 6 other XCM races throughout the year, typically defined as B races with openers.
I specify XCM as the discipline for the 1st six months and then last year I chose Gravel for the 401km one (prior to that I always used Gran Fondo)
For my year end race I’d start doing longer rides on Sunday (I specify them as Solo in Plan Builder) on my calendar. Saturday rides are the normal 90m interval rides. The Sunday rides max out at about 4 hours. For me personally I find that you definitely don’t need to do these endless long rides to prep yourself to complete a ultra endurance event - but other people might have a different experience. Of course if you want to compete at the sharp end then it might be a different story also.
I guess the most difficult part for me in these long events is to keep on continuously fueling (I try to do 90g/h CHO) and based on my experience the reason people fail in these events are not fitness - you can go very long and very far if you go easy. Anecdotally it seems to always be fitment or equipment issues and not fueling properly.
FWIW I’m 45y old and sit at around 3.8 w/kg, so definitely not an elite type athlete by any stretch of the imagination (despite my best efforts 
).
Nutrition is definitely the one that catches many out at the longer durations. They assume that what works for 4-5 hours, scales up to 12, 24, 96, 100+ hours and are a little shocked when they find it doesn’t.
This is great. I thought I needed long hours, as in 20-25 hour weeks. I was guessing this would be closer to training for an Ironman, which is my closest experience to a 11+ hour long day (yeah I am slow). Now I feel more confident even though I don’t have much time to build up to 20+ hour weeks. Thanks!
ps: 43 year old here.
This weekend I was working on my nutrition and failed miserably. I am at about 80 g of carbs an hour and wanted to push closer to 100 but forgot about the uptick in my bottles and ended up drinking 120 g of carbs an hour. That didn’t go very well but only noticed close to the 3 hour mark. It takes a while for the gut to settle. Luckily I always carry most of my water in plain form.