If you have a power meter, you should use it to determine your zones, rather than HR. If you can do 20 minutes with an average HR of what you are calling zone 5, then that's not a hard enough effort to qualify as "intensity" in the type of polarized training that is described in the video you linked to. That's a threshold effort by definition, exactly the middle-ground the polarized model says to avoid.

He describes lots of time in Z1 and Z2, literally talking about skiers walking up hills rather than skiing, and time at vO2max and higher intensities. That kind of intensity you can sustain for 4-5 minutes, maybe 8 minutes at the absolute limit, but really wanting to die immediately afterwards. If you are "super comfortable" or any kind of comfortable at that level of intensity, you are not doing it hard enough.