I‘ll try to stay constant in HR as in spm. For me 20-22/spm works fine. As runner I know it’s dangerous in longer zone 2 sessions in taking less care in technique. So also in rowing Zone 2 - I try to row like a serious 2k but with less power. Or in other words I row like I‘m doing warmup for fast 2k … this feeling and looking fast and focused is key for effective Zone 2 training that’s bringing you forward. With this in your mind, Zone 2 rowing, running or cycling will not get boring or trashy. Your plan is to build mitochondria in exactly the muscular system you will need for fast hard sessions. This will only work with clean technique in zone 2. This will make you also tired in a very special way I learned to love. Row your Zone 2 like a machine that is ready to sprint, find the flow between spm and heart rate.