yep that’s the key isn’t it, more for less and with minimal stress. So in your event when you need the effort, it is there because1) body has not been under stress and 2) Glycogen stores are largely untouched.
So fpor example when I did my 305km ride and nearly 5000m of climbing the average HR was near 150 for 12 hours, no way as a carb burner I could have done that, no way. During this ride all I had to eat was jelly babies, that’s it, so that just shows the fat burning levels are way higher that ‘zone 2’ and goes as FASTER showed into Zone 3,4 and possibly 5 still burning more fat that carb.
Of course what should be remember is in ultra long events you can see use carbs but not replace it, if you are doing most of your effort at ‘zone 2’ this means your body through GNG has time to replace any glycogen being burnt on demand. The problems happen when this replacement can not match what is being used so then you have to supplement by external carbs whether drinks or food.
In essence a human if they stay in zone 2 can do that for a day etc. Now what is interesting is I know nan ultra marathoner, he for example has done 10 in 10days. However he swears he is just a normal carb burner. However when I look at his stats (he is only slightly older than myself) he is comfortably running in zone 2/MAF for the majority of his runs. He is so finely tuned that will be fat burning without the need to go into ketosis but he has to supplement during these runs because his body is not efficient at the swap over points which will be lower for him than us.