I’ll step in first – rode and finished in 2013 when it was only 420miles.
Ride your MTB a lot. Ignore your road bike. Ride your MTB. Days need to be back to back. Loaded. With little food. Ride more. After a while you’ll want to stop. Don’t. Ride your MTB more. You need to be technically good, like really good, especially when you’ve had no sleep-food-water. Hike, lots, with your bike.
The bike you ride, and the kit you carry mean nothing if you’re not fit. So ignore kit fetishising and train. Be fitter than you have ever been. Stronger than ever mentally. Then do it.
You may fail. About 50% of people do.