Building up distances from 0 to 300 during events is the basics. Most longer organised rides (non Audax) I have done had an early morning start. So this goes along with my normal bio-rythm.
If you have a 400km. I devide this in a morning ride 200km, afternoon ride, 100km and an evening ride 100km. It's all a mind set, don't let the big numbers frighten you.
If you do a 600km, it's nothing more then riding 2 times a 300km, with you are familiar with. I normally book a B&B half way so thay you still have a good night sleep. I don't like to ride during the night.
If I do rides longer then 2 hours, I use BORN sport nutricion. My Garmin gives me every half hour an alarm signal so that I don't forget to eat and drink. Because your body has a limit of carbohydrate intake a hour. You want to maximise this, so that you don't run out of fuel. Bigger water bottles (2x 950ml) is more weight but you have to stop less to refill.
Also training your self to have short brakes during the ride, will make it easier to finish your ride within the time limits.
Shorter distances are a great way to build up some speed. You may benefitt from this on longer rides, because if you can drive faster, You have less time on the bike and more time for recovery (at (sleeping)brakes).
Also do some training on hills or mountains of against the wind if you expect to get this circumstances on your ride. Do some investigation about your ride so you know where the controls are (google maps is very helpfull) and you know in headlines where the route takes you to.
And ofcourse have fun!