I actually love both - carbohydrate as well as ketosis. I can do long grinding tasks by eating loads of carb as well as by long starvation (of carb).
So, I feel that it does not have to be either carb or fat. Body can be trained to switch to either fuel at will. Just to start with, I go without food for 16 hours everyday (from 8 pm dinner till 12:30 pm lunch). Then I eat heavy lunch and dinner - carb mostly.
How do I know if I'm switching to ketosis? I assumed since I can be active without carb this long. On an extreme case, I had gone without food for 48 hours during NEF enduro (except for two buns and little dry fuits) and I was strong till the end though I was extremely hungry and sleepy at the end of it.