I'm doing some very low intensity "training" (it's so low intensity, I hesitate to call it "training") which includes a long fasted weekend ride; in conjunction with a very low carb diet - less than 50g a day.
Last Sunday, I managed 102km completely fasted, around where we're going to be living very shortly (and where even the flat bits have hills - at least, for this flatlander's legs) without any energy issues. Sure I was VERY ready to eat by the time I finished, but I didn't need any carbs (tea was cheese, chicken, ham and a fruit salad at Wetherby services - classy) and energy flow and focus was easily good enough for me to drive 300 miles back south in shitty weather, without any problems whatsoever.
Get the fat burning right - and you simply don't need to worry about food. Or digestive issues during a ride, because you're trying/having to force food in. But to get it this useful, you have to follow what most people would regard as a really unbalanced diet (75% fat, 20% protein, 5% carbs). No bread (no grains of any kind) and no sugar except small incidental amounts in fruit.