Some things that correlate with my experience:
The 70% of HRR (Heart Rate Range) is a good marker for where the Recovery Zone ends
(training much above this for any length of time means you're not recovering; recovering here means rowing/training gently enough to be able to train hard the following day)
Before I learned to train by HR, and prioritise active recovery, I used to over-train on the recovery days, and end up losing performance over time.
When I learned to recover properly, I could sustain far more overall distance, still train at least as hard on the hard days, and improve over time, and I set a load of PBs as a result.
I tend to row at constant pace, unless I've got the pacing totally wrong. For me, when I'm doing HR capped, I try not to exceed the cap, but resort to Av HR staying under as a last resort.
But I'd avoid 'pushing it' too much, as it's obvious you might fail to recover properly.
I also quite often will use stepped paces, (row at higher, then step down) and if my HR is high (exceeding target), I'll step down earlier. This way makes it easy to adjust on-the-fly.
There's a limit to how far you can go, even at recovery or below and still count as recovering. When untrained, rowing a HM or FM will obviously not work as recovery, even if you keep the HR lower.
Once you get trained/conditioned, it can be surprising how far you can row and still be recovering OK, but clearly you don't want to be right on the limit all the way.
When conditioned, I will use HM distances at lower intensity as recovery day sessions, just won't push it too much.
Sometimes I might even use a HM at near threshold on a recovery day, especially if the HARD days are relatively short distances.
Sometimes I might do a minor short sprint (at the end of long steady state) on a recovery day just to mix it up, but you have to be careful.
Listen to your body (although be prepared to ignore it, if the HR is nice and low and telling you you're OK, even if your head doesn't think so)
Consistency is everything for me.
When doing long steady state, you get a lot more HR drift when it is hot - it is an obvious side-effect of dehydration. If you see a relatively sudden increase after a long period, it usually tells you you're getting low on fluid (or possibly carbs).
When well conditioned (and it's not too hot) I can sometimes do long distances without any HR drift. I sometime even get an inflection (drop in HR between 20 mins and 60)
The anaerobic threshold does seem to be moveable (for me, anyway), rather than linked to a specific HR %age; if I train at high intensity a lot, I become a lot more tolerant of high HRs, and able to sustain them a lot longer before having to back off. Conversely, if I haven't done any high intensity, I'm unable to tolerate the high-end anywhere near as well (although it also tends to mean you hit higher Max HR in sessions). I've also seen a lot of HR data suggesting that high-end athletes can sustain very high HR for amazing lengths of time.
There's probably more, but I have to finish now...
5'11", 50 - older, slower, greyer...