I think that all makes sense. A few things:
* Heart rate is not really a measure of training intensity; it’s a measure of stress. So the speed you’re moving, and the muscular load is only one factor. The length of the event–and the amount of muscle damage–is another. As is heat and dehydration. As are start line jitters. Heart rate really only measures training intensity when all other stressors are absent or minimal. So although your speed in an event that long is probably mostly Zones 1/2, the stress could easily be in Zone 3.
* And with an AeT/AnT spread that wide, it makes it even more likely that your heart rate would fall within that range.
* I don’t know if altitude is a factor in AeT versus AnT. Usually, the higher we go, the less anaerobic response we have.
* To be sure, I would get a lab test done. Once you have that, you can skip the drift tests, even if the lab tests are done much further apart.
* With all that training, I suspect that your AeT may be higher than you think. At what bpm can you comfortably breathe through your nose?