First of all I don’t think it is possible to say one session is better than another. Endurance is built in blocks, considering the the average half live of adaptions 3-4weeks blocks (source: again Coggan).
I don’t know if one has to ride at fatmax to build fatox (especially at higher intensities). All I know:
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Bu says Blu was carb thirsty. He did plenty of volume <LT1 and this shifted fatox at race pace to the right
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training volume (no necessarily duration) is one of the few parameters where we really see a strong correlation with mitochondrial content. And the more mitochondria, the more fatox. However, we also see an increase in intramuscular fat with high volume. The fat glycogen so to say. Close to the mitochondria. Adds to higher fat ox, the substrate is there.
Type I and IIa fibres in two different muscles:
I don’t know if it is volume at fatmax or just volume. However, I don’t really know if this is relevant since the “higher fatox zone” simply coincides with an intensity level where one can accumulate a lot of volume without overdoing it. Too much can impair recovery. Impaired recovery equals less adaptions. At some point there is no subsitute for volume. As WW has said, the number of muscle contractions are crucial as well. These trigger vascular adaptions. Supply lines for your mitochondria.