Ventilation is the best real time indication of your metabolic state for field testing. Increased ventilation depth and rate indicates you are needing to blow off more CO2. As mentioned in our book and an several articles on the website that Aerobic Threshold and entire aerobic metabolic response to exercise varies daily depending upon recovery status. Having done a long session that no doubt depleted you glycogen stores just 12-16 hours prior had the obvious effect of lowering the AeT HR substantial. Basically, you had not fully recovered from the previous session.