I am unable to train at any other zone but 4 or 5. When I jump on a bike, I have to spin. My cadence is around 70-80 and that take my HR to at least Zone 4. I'm 41 years old so that means around 145-160 bpm. I want to train to do longer distance but I am uncertain how I will achieved that. I would have to reduce my cadence in the low 50's to get low enough and that would reduce my speed to below 20km/h.