Harry;
If you were forced to cycle daily on hilly terrain you’d handle it just like if you were running on same…..slow down on the uphills to stay in the appropriate intensity zone. As you basic aerobic fitness increases you’d be able to ride/run faster for the same HR/metabolic intensity and would not need to creep in to Z3 on the uphills. The only reason your HR creeps up into Z3 is that your aerobic capacity is low enough that it can not supply all the energy needed to maintain that pace going up hill so it has to supplement ATP production using your anaerobic metabolism. As your aerobic base improves you will need less and less reliance upon the anaerobic pathway.
Scott