I think it can be that hard, I have been on two opposite sides myself:
- couple years ago, my TTE was 70min at 98% of FTP, but when attempted 4x8min at 105%, I could complete only 3 intervals and 3rd already felt like all out
- but, during current spring, after max sprint block (e.g. 5x30sec all out), I could complete 4x8min at 112%. Didn’t measure TTE at that time but I believe it might have been around 30-40min.
Don’t know the reason. Anaerobic contribution certainly improved a lot, also lactate tolerance but I am not sure how much it would help during so long intervals 