One thing I must stress is that you need to get your ass in shape. Not your ass as in “I’m gonna nail your ass”. I literally mean your ass, because nothing is going to kill the enjoyment of your ride quicker than some nasty saddle sore (imagine riding 50 miles on a giant irritated pimple) or other chafing misery. The skin over your sit-bones and your saddle is the primary interface between you and your bike. Get it good and tough before your long ride, and get some nice cycling shorts. When I was 20 a buddy and I rode ~300 miles down the CA coast over a week wearing gym shorts, t-shirts, and sneakers (but still had helmets and biking gloves) but I wouldn’t recommend that. It was more out of ignorance than anything else. But because my butt was in shape I still had a fantastic ride.
Bike shorts and moisture-wicking jerseys are expensive but they’re really worth it.
One consequence of having your seat too high is pulled or strained muscles. This is another thing that can kill your ride. It may not be debilitating, but it’s still painful and you’re reminded of it constantly.
Also - and this is hard - is figuring out how much water you need to haul vs. what’s available along the way. Water is heavy and you don’t want to haul more than you have to. At the same time you never ever want to run out.