If it were simply a flat road marathon, I would say using ketones in a fasted state would totally be an ample form of fuel but since you are running uphill, I think your body might prefer glucose (stored glycogen) at least some of the time.
You could try to store some glycogen in the day previous to the race by eating a few more carbs than usual, after a good workout. Thomas Delauer says this about carb backloading:
Idea Behind Backloading:
The idea is that you use insulin to your advantage by not eating carbs when your body is most able to store them as fat (early in the day) - instead, you eat carbs when your body is most likely to store them as glycogen in the muscles (later in the day, after working out) The post-workout point is important: you use weightlifting later in the day to deplete glycogen stores and increase insulin sensitivity in the muscle cells, but not the fat cells, so when you then start eating carbs, your body preferentially shuttles them into the muscles, not fat cells. The theory is that insulin sensitivity will be increased in your muscle cells, but will remain at a lower sensitivity in your fat cells. As a result, when you start eating carbs, they will instantly be utilized by your muscles, rather than being stored as fat (1,2) Good in theory, but you’re not going to see a huge difference in insulin sensitivity if you’re already carb depleted
What’s also comes to mind is that while you do generate lactate your liver is able to match muscle production with clearance via a process called the Cori Cycle (a process in which the liver turns lactate back into glucose). I’m pretty sure lactate will be in good supply during this run but I honestly don’t know how it may or may not apply to your run.
It will be interesting to see how this goes for you. Would it be possible if you could do some dry runs to see how it affects your energy levels.
https://peterattiamd.com/the-interplay-of-exercise-and-ketosis-part-i/
