Zone 5 is usually 1-2 minutes out of a longer 30 minute cardio session for me, I do it as a final sprint. I am not talking about repeated hill sprints where you would get 10+ minutes of zone 5 cardio in a session, which I agree would not be something a normal person should do 4x/week.
From my own experience, it seems like hitting that Zone 5 briefly is a good nervous system reset (overrides any dysfunctional breathing and heart rate effects from long covid); it's less about training the heart, although that's an excellent side effect.
