A very rough outline: A real training plan calls for different workouts each week.
While training for a 50 miler, riding "as much as possible" without structure is a good way to burn yourself out or plateau at a level below where you'll want to be, so don't do that. . Most of the races I do are 50 milers, 4-6 hour races etc.. Depending on the time of year, and how much time I have, a typical training week consists of one, maybe two long rides of 3-5 hours, which should be done at an easy(ish) pace to build endurance, a couple of hard and varied interval sessions (45 - 60 mins) and one medium length (1.5-2 hrs) ride, hard, without structure. You will need these harder efforts because even though the average pace will be lower than an XC race, there will be plenty of times when you'll have to go hard, like on the start and the climbs. The long base building rides are more important in the months leading up to your target race, add the intense stuff as you get closer. Make sure you taper off the week prior to the race; ride to keep your legs loose but don't push it; you want to show up on the start loose, rested and recovered
Training is different for everyone, so YMMV.
The same holds true for nutrition/hydration...you'll need to find out what works best for you. Find something, or a combination of things that won't upset your stomach, because that is a race killer. Personally, I use gels exclusively for races of this length, (about 1 per hour) many other racers I know need more "real" food. For hydration I alternate between water and energy drink. I like Scratch Labs or Powerbar Endurance, unlike some others (Cytomax, Accelerade are too sweet, and Heed tastes like a camel's rectum) are light, not too sugary, and taste good.
Best of luck!