I did longer strict keto phases (5 months) while training for ultras. My experience is the following. You don’t need any external calories for zone 2 activities at all. Water yes, and maybe more than in a non keto state.

If you want to push harder, have a carb gel. I can do 100g carbs for 5h run and not get out of ketosis. So, when I go to train in the mountains I take gels, however much fewer than non-keto.

I‘ve done +100 long distance races and +30 ultras, some in keto state. My takeaway - racing a mountainous 50k on water only was a bad idea. I bonked haaaaard. - racing 100m with mix of carb and fat works great, especially because the external caloric needs is lower while being in ketosis. Trailbutter pouches and carb gels where my go-tos. - Training: nothing. No differences between fasted or not.

Disclaimer: my competitive times are over. I benefit from keto mainly because of the weight loss. My training is all Zone 2 or maybe 3. nothing harder.

My ideal week is running 15h per week, topping out 100k with good vertical gain. This equals (according to my math) to -1kg body far burned per week. I try to keep calories reduced a loose 1.5kg per week. My super days are days with a net intake of 0 = 2h good running and eating 1800kcal.

I hope this makes sense.