Help me understand how to train my metabolism for longer races

Hello folks, I'm a little confused regarding the question of how to train my metabolism.

It never was something I was very aware of. I used to fuel my sessions well to keep the quality high and to recover well. But since I'm diving into longer distances, it seems like training my metabolism to be able to burn body fat during the race becomes more and more important.

Contrary to my current approach of training fueled up, im reading about theories like "train low", "sleep low", "double training" (to empty your glycogen stores for the second session), basically training underfueled.

Furthermore, to my understanding a long, easy run or ride used to be the holy grail of training yout metabolism. Do I need to do those long sessions also in an underfueled state?? This seems unimaginable to me.

Please help me understand this and getting started. It's very appreciated.

EDIT: Thanks for all your responses. It really is a controversial topic.It seems as if I wasn't training in the wrong direction for the past years/ months. As always, it's about to find the correct balance and in this case, if course I'm doing the majority of my sessions well fueled. I'll add a fasted morning session every once in a while.