There are two different aspects to this:
(1) You start out using glucose as fuel, then switch dynamically to using ketones - this can be difficult to impossible for some people. You can have more initial energy but less stamina. The switch over point can be harsh.
(2) You start and remain in ketosis - that is you have been eating very low carbohydrate so that your body has to produce ketones for your basic daily energy requirements. This IMHO is a better way to operate. You have less initial energy but more stamina. You don't have the pain of transition between energy modes.
I have read that there is a point where your gut cannot take on board enough food (even gels) to replace the blood glucose being used and that is one cause of the "bonk" in riders who keep hydrated and fed. If you are already burning body fat then you don't run out of fuel.
When I'm cycling fit and eating LCHF I can cycle reasonable distances on just a cup of coffee with butter and double cream.
That is, at least 1-2 hours which for me would be 20 miles+.
Cycling companions are desperate for cake after about 1 hour.
There are references such as :
https://www.amazon.com/Art-Science-Low- ... 0983490708
https://www.amazon.com/Art-Science-Low- ... 983490716/
which are quite old now but lay out the basis of Low Carbohydrate living and exercise.