The problem is how to do it. Rowing is not like other sports. It's a power sport that uses as much muscle as possible and fast, since the idea is to shift a boat. So if you pull a decent stroke, worth say 8W-min, even at rate 20 (so 160W) you'll be over UT2.Digging deeper into this it seems there are benefits of rowing for lengthy periods in UT2
To keep HR down in long work, better go for fastish walk; if no hills, HR will stay down, as you know already.
So on erg try warmup plus 15-20 minutes a day hard and fast: rate 23-24 and average 200W. See the C2 WODs, which are specific to rowing.