Davide, If you want to move the needle on your AeT then you certainly need to prioritize the percentage of your training that’s is in Z1/2. Whether that is done in flat or hilly terrain is a different question. What kind of terrain do your goals have? Are those goals going to require a running gait exclusively? How much time do you have to prepare? It is very very common that folks are unable to use a running gait uphill and stay under AeT. This is just part of the deal, walk those sections and run the others. The reality of pretty much all mountaineering/ultra running events is they’re going to involve some or all walking so you might as well train in the way you’ll need to perform.
As to your example of 69% Z2 and 26% Z3 I would say that’s heavy on the Z3 if your exclusively trying to improve your sub AeT pace, assuming thats reflective of the way you train every session. That said if that’s just 1X per week that’s a different story.