Intervals can make you strong. Sprints can make you fast. But only adventure can make you tough and determined. Sitting on a trainer grinding out the hours requires a certain mental fortitude (one I'm not equipped with) but pushing a loaded bike up a rocky quasi road in the rain, four hours into a ride builds a real world resilience that indoor workouts cannot match.
Now to be perfectly clear, I do intervals. I have a power meter and even wear a heart rate strap on most rides. I do sprints and a few times a year I grind out the hours on an indoor trainer. Training smart has its rewards. But it's when I'm outside, managing my exposure to the elements, that challenges me both physically while also tasking my problem solving skills. Working out the on-bike logistics that a long ride requires is much of the fun for me.
We all get them wrong from time to time, but hopefully we also learn from those experiences. It's easy to be once bitten, twice shy and this sometimes leads to overcompensation. A good friend who helped introduce me to bikepacking and ultra racing says, "we carry our fears." He means that if you fear the cold, you'll probably carry too much clothing. Is the dreaded bonk your worst enemy? This makes overloading on food a possibility.
After years on the bike, doing long events you come to better understand the risk/reward, speed/comfort relationship. The give and take of those aspects of cycling can be tricky. They can even change from one event to the next, one year to another. Those ratios are highly personalized too. What works for me will not, necessarily, work for anyone else.
A trap that I once found myself in was reading account after account by other people about a particular event, the Tour Divide. Now, I certainly gleaned some great tips and tricks. It helped to challenge my approach to it all. But like all advice, you need to consider the source. And if you don't know the person doling out the help, it's hard to gauge its usefulness for you until you go out into the world and test it.
So get out there. Don't be afraid to be a little uncomfortable. Take precautions, but a few nights of bad sleep only a few hours' ride from home is quite a bit better than three weeks of bad sleep because you didn't do your homework. If something went wrong, adjust it. Take notes if that's your process (it's certainly mine). But also get out there with new people. They can teach you and you them.
There is no substitute for experience. The doing of something is the only way to actually learn it. Thinking about the doing, brainstorming the possibilities can be helpful. But ultimately you have to go outside and figure it out.