> As you know, carbs are precisely we need when going the distance.
This is completely false. You deplete your carb stores after a few hours (depending on pace etc), and then YOU ARE RUNNING ON KETONES, whether you like it or not. You physically cannot eat and digest enough carbs to sustain your pace for any longer period of time.
Carb depletion can be a real shock for the first time. What happens is that your body is not accustomed to burning fat for energy, and so it just doesn't know what to do.
But if you prepare it will learn how to do it, and do it rather efficiently. Unless you are a pro athlete (in which case you have your own dietician anyway) you will benefit from going keto for long distance endurance sports. Yes, your sprints will be slower, but in exchange you will simply not be hungry, or feel loss of energy (actually, a marathon is not really that long distance, if you are fast enough you won't even deplete your carb stores!).
It takes a few days, maybe a week or two to switch to keto. You can speed up the process by fasting, it will not leave your body any doubt about what is going to happen :) You will feel very weak at first, but this will pass, and then you are in long distance nirvana. You just need to watch out for proper hydration (not too much, not too little) and mineral replenishment. You can even do your sport fasted, it won't make any difference! Highly recommended for those having digestive issues.
As a matter of fact, I'd rather say it is highly irresponsible of anyone doing endurance sports NOT on keto. But as many other things, this is not taught at school, and people have to experiment for themselves to find out what works for them.