I don't know anything about the event, but the name implies you may be riding at night, or through the night. If so, then training yourself to ride in the dark (testing and getting used to equipment) will definitely help. Knowing when you need to stop and nap is also important. I've ridden through the night 3 times for 600ks and a fleche, and each time I had very similar timing and manifestations of stupid sleepy.

On the mileage part of it, Indy's got great advice. I think a lot of times people really overthink training for endurance events when it's best just to build up your saddle time in a rational way.

Sounds like fun!