I can tell you what I do, not scientific, but works for me. for durations of 4hrs+, I train the way I race, i.e take in nutrition before and during efforts. for training efforts 2hrs or less, I do not take extra nutrition before or during, but I always eat some protein within 45 mins of completing the effort. if I am racing 2hrs, I will drink a Maurten or Beta Fuel drink before the race.
all of that said, I definitely get a positive energy bump if I take a carb drink 30 mins before 2hr training efforts…I just generally don’t do it.