@Ludo - first question is how long is long?
Your body stores energy in the form of glycogen. The stores about 400 grams which is approx 1600 cals.
The maximum rate of carbohydrate consumption per hour is 60-75 g. Source: http://cyclinguphill.com/energy-consumption-whilst-cycling/
So if you have been eating well and your glycogen stores are full you can cycle for over 5 hours without any additional energy.
However, if your stores are not full you will be some “Fast” energy, banana, gels, etc.
I did a 26 miles (1.5 hours) ride the other day and has a Huel (100g) before I left home, and about half again on the ride and felt good.
More reading here: Huel for ultra marathon running?
Also have a read of John’s tweets - https://twitter.com/ultraover40
He does ultra marathon running while living solely on Huel.