I’ve just done a 110km event having not done any rides longer than 55km this year.
I should have done longer rides in preparation, my legs were empty after 80km despite fuelling well and I was starting to get twinges of cramp after 100km
I should have spent more time in the gym, strength and conditioning work is important when holding position on the bike for hours on end.
Have a look in the Audax thread. Lots of good advice there.
Basically, 2/3 intensity sessions per week give you the engine and help you know your zones.
You can get a century done without too much trouble, without doing longer rides.
For anything double-century and more, you need to get your fit and nutrition dialled in, know how you feel on the bike after 4- 5 hours, and do a good amount of strength training to perform and mitigate the damage.
You will discover a lot about yourself on those rides, and it will be hard but highly rewarding. Enjoy and let us know how it went 