I read somewhere that on long runs of up to 18M it's better not to take on fuel. That way you force adaptations in your muscles allowing them to store more glycogen. I can understand this principle, however if on runs above 18M you do use gels/drink, what is the benefit of storing this extra glycogen? I can understand in the days prior to gels you would want to push "the wall" further away using this technique, but nowadays is it still relevent? I actually ran a 17M long run yesterday without fuel. It was not particularly comfortable due to the heat but I did manage to run run it ok, with no slowing down towards the end of the run. I suppose it shows you can save money at least by not having to buy gels on these medium-long runs. But is there another benefit to depleting your muscle glycogen?