I think a lot of it comes from years and years and years of riding. It also probably helps that my favorite rides to do have always been long ones. Doing several stage races where you ride hard day after day after day, gets the body used to recovering.
Basically it comes down to time in the saddle. Many, many hours spent pedaling around will get your body, mind, and butt ready for something like this. That’s not to say you need to wait until you’re “perfectly” trained for an event like the Cohutta Cat. I’m a big proponent of just getting out there and doing it. Bikepacking is very different from other forms of mountain biking. As long as you eat enough, drink enough, and take breaks as needed, you can ride for a surprisingly long time.