I agree with the "window of opportunity "concept immediately post workout where simple carbs can be ingested and preferentially shuttled to muscles to be stored as fuel(glycogen)however that is not to say glycogen stores can't be refilled at other times with complex carbs after normal digestion.In fact if fat burning is the goal and you are not totally depleted of glycogen then you will continue to burn fat postworkout if you don't immediately reload the carbs.It all depends on your objectve and I mention this because you previously stated weightloss as a goal.I disagree with the second part of alex's statement again if weight loss is the goal which wasn't mentioned in this thread.My understanding is for aerobic exercise the body first uses blood sugar for energy and when this is depleted (maybe 1/2 hour to 45 min.max if full to begin with)the body switches to a combination of glycogen(muscle sugar) and stored fat.This is the first hitting the wall I referred to in the other thread that you can train through and should if you want to burn fat .My understanding is the percentages of each used varies with the intensity of the exercise with low intensity favoring fat and high favoring glycogen.You have maybe 1 1/2 hours of fuel stored as glycogen(again if full to start)and if all glycogen gets depleted the body goes into a crisis mode(second hitting the wall which most refer to as the "bonk")and cuts off fat burning and starts cannabalizing muscle to get protein for fuel.You can even train through this but if you want to retain muscle and burn fat not train to win a marathon you don't want to.The point to all this is if you don't keep topping your blood sugar with simple carbs you will burn a large % fat with long slow to moderate paced rides.However if fat loss is not the goal topping up the blood sugar is the most efficient way to go because it is the bodys oreferred fuel and most efficiently burned.