I was amused at the thought of riders conscientiously snarfing sweet goos or potions for fear of bonking on a 20 mile ride. While I have no interest (except perhaps academic) in low carb diets, I have for decades eaten very little during the day, and getting most of my nourishment in the evening. I've read in many places that this is bad, but that's what I do -- usually large coffee or tea in the early morning, sometimes with sugar and milk, sometimes black; a small snack in the early afternoon; dinner after 5 pm.
Anyway, while I don't ride long distances, I have no problem riding a vigorous 30 miles on coffee and perhaps bread and cheese (coffee only if before 1, perhaps a snack if afterward), and I don't bonk, unless I wait too long after I return before eating something -- I will bonk if, at say 3 pm, I return after a ride on an empty stomach, and don't get around to eating because I get busy with something else -- for situations like this I do carry an emergency stash of glucose pills.
The problem with riding later in the day on an empty stomach, while of course drinking a lot of liquid, is that you need to pee every 15 minutes, which raises eyebrows.
Patrick Moore