Thanks for writing in with your questions. I’m assuming you ran or skied back down the 1000m hill between laps.
Your HR drift upward indicates your aerobic base is currently inadequate for this type of training, making this an “overreaching” workout. That’s not a terrible thing to do occasionally, but pushing into Z3 for 2.5 hours Saturday and another 30 min on Sunday made this a really demanding workout that was not doing what you need it to do. Your aerobic capacity deficit forced to make up that deficit by utilizing the glycolytic metabolism to make up the difference. This caused significant metabolic stress and if continued for a few weeks will lead not only to a lowering of your aerobic capacity but very likely to over training.
For this week you should definitely drop ALL intensity from your training and focus on Z1-2 work only.
Going forward you need to reduced the intensity of this type of workout. You can’t force an improvement in aerobic capacity to occur faster by training above AeT. In fact this will have the opposite effect and drive AeT down.
I hope this helps.