Didn't see this before! Although the thread is roadie based and I'm a mountain biker I've got some general stuff to add:
1. Fueling wise I'm a huge fan of Tailwind nutrition. I did a 12 hour last year using it as my primary fuel source and a 6 hour just a few weeks ago with it. I don't use ONLY liquid drink like they say you can and some do. I carry some small amount of solid food, like fig newtons or costco nut bars. But the vast majority of my calories on these rides is Tailwind. I usually go 3 scoops (300 calories) per hour. I drink from a 3 liter camelbak and usually around 22oz per hour, so I can easily calculate how many scoops to put in to start. Then I have the rest pre-portioned in double ziplocs that I dump in when I re-fill water.
2. Route planning is important. For instance with the above fueling I have a 3 liter capacity in my pack but that's a lot of weight to carry if there is going to be a stream or lake to fill up in halfway through the ride. Obviously it's not just distance that matters but also how much climbing/descending there is and how gnarly the terrain is. You migth ride 20 miles in 2 hours and the next 20 takes 4 hours.
3. Tools, it goes without saying you'll need to take what you might need to at least limp back to a trailhead/bailout spot if something breaks. I carry a nice crank bros bike multitool, tire levers, spare tubes (1 or 2 depending on how remote the ride is), spare chain masterlinks, spare tubeless stems, first aid kit, tire pump and shock pump. And zip ties! Edit: I also carry a sawyer mini-filter, because giardia sucks.
4. Training, I'm a big fan of getting in consistent training with a HR and power meter because I have a real tendency to push too hard during training and riding. In endurance riding this is dumb, you need to go long and being a tuffguy is counterproductive. I love Zwift on my spin bike. It has structured training programs built in, or you can just do a ride from another plan and hold the power, time or HR you know you need to. The point is, it's on the screen the whole time you train, and I can do these rides regardless of the weather or my sometimes hectic schedule.
5. Mental training. This IMO is a massive factor that should not be overlooked. ultra distance/time events always boil down to being tough, doing things your body doesn't like, things going wrong, the voice inside your head telling you to bail and so on. I'm a big fan of David Goggins' as well as Rich Roll. They have different approaches. Goggins is about being hard, introducing discomfort into daily life and ramping it up to train yourself into being used to it. Roll is more about meditative style, being in the moment, accepting where you're and doing your best to make that next tree, that next rock and so on till you're through it. I use a blend of both and feel like my mental game is really on point, if anything I lag behind on the physical training, haha!