I don't think OP is asking what to eat while riding, but what and when to eat during the day before riding.
Some people don't like to eat right before riding, or even up to a few hours before riding. I figure if I eat while riding then eating before riding shouldn't be a big deal, but you want to eat foods that are easily digestible. Also goes for food while riding.
I would definitely agree that it depends on intensity as well. I am more sensitive to what I eat before doing intervals than before a long endurance ride.
As for intensity, Im old and fat and trying to get back in shape. Me going uphill is like a tractor pull... everything is intense.
SO after an hour or so, I start to wane, "bonking".. I think its called.
I was just wondering about eating for endurance. I think maybe I should just keep riding....