The first question to ask is why to fuel ? The second question is when, but that is partially already answered by the response to the first.
Normally, during the base period, you build your long run. You also build your total volume. This means a gradual increase in distance. This is a time when you work on an actual ability, not a race specific feature. You go from 15km to 35km in a period of 3 to 5 months, for instance in case of a marathon. You build the muscular and articular ability to last easily the distance. During this period you might take in some calories or have a light breakfast before, but in reality, you would not need absolutely anything. Maybe plain water.
Of course, you should be able to properly refuel and rehydrate during your day and should be in a good state before that long run too ! That is obligatory !
When you enjoyed yourself out in the mountains, going light and fast, that is the same. I like to be outdoors, absorb the nature and not to worry about my human needs. I often spend 3-4-5 hours with no food and no water going on, at all. In mild weather, often with not even a t-shirt.
However, when specific needs come into the picture, intra-workout fueling is a crucial element to follow. Actually not only during long runs, but certain interval workouts to. When doing a lot of intervals like 4 x 2000 on 150m/R50m style, followed by 3 x 1500 @ threshold for exemple. I might squirt a micro dose of sugar under my tongue after each rep. The mental presence and the 100% focus is helped and stimulated by glucose. Not necessarily the actual energetic system what is boosted, but the brain itself. These workouts are in the training-cycle to be completed exactly how they are prescribed. Execution is key and the precise pacing is what will aid the athlete to be better, not the metabolic effects.
It is like running at lactate threshold heart rate or running at the so called lactate threshold pace. One is training an ability at the end of the base season and the other is preparing for the race itself and while we might call that LT-pace, in reality it is just a race specific pace-range.
To explain this more precisely. During the final weeks of the 1st macro-cycle, you make an LT uphill session guided by your LT Heart rate. Warm up, then when running uphill, in about 60 seconds you are at the prescribed HR and hold it for 35min. You just increased your threshold. Threshold is the ability to produce lactate, resist and clear lactic acid.
Then, when in the second or final macrocycle, you’re 10km race pace is 3:30/km. Your LT pace is around 3:45/km and you play around with it, doing tempo runs and long intervals. This pace will be gradually dropped towards 3:36 to 3:38. You will ignore HR for these workouts.
In case of metabolism it is the same. In some workouts, you just don’t care what the actual metabolic effects are, what sugar and fat utilisation ratio you are at and so. Especially true to 10 mile and under. For a half marathon and definitely for a marathon, you have less ground to play at. Over-fuel and be loaded all the time and your race can suffer from the constant sugar intake. Under fuel and your stomach will most likely get upset due to the lacking habit of eating during effort.
In case of the long run, especially when we talked about the marathon, fuelling is very important. Once you start injecting in faster paces, like m-pace or even faster intervals, lasting 35km is not easy.
You should not over-do it and definitely not in all instances ! Some athletes take in more fuel during a long run, that they would do in the actual marathon race ! Bad idea, making you much too reliant on external sources of nutrition. Some long runs might have fuel only in the form of breakfast and you might take in exclusively salt-water; maybe some amino acids. Maybe you’ll be taking in a gel every 30min during 150minutes. That would be a test of the stomach too, to see what you can handle at those paces during those longer time periods !
The benefits of modulation of training is true to fueling too. When an athlete is able to take in only 4 gels and only from one single brand and only from one flavour and only 38 minutes apart, well that runner becomes very vulnerable to everything. Weak ! That is most likely the type of athlete who also stocked up 25pairs of running shoes from the same brand and the same model, as that was the “best for him” !
You should be able to run, with or without, but when it is with, your presence and performance should increase !