So long as you maintain a high work level in each single stroke, you are getting a good workout, whatever the rating. Stroke Work is Watts/Rating.My main challenge is accepting that fact that rowing slower for longer distances doesn't mean that I'm not getting a good workout. male, 67 y/o 6’0”. 5k PB is 23:49.
From endurance and total work/heat generated points of view, the lower the rating the better, because you can go much longer distances.
At your age and height, I suggest your stroke work target should be about 7 Watt minutes, a level that will guarantee you a good workout at all ratings down to 18.
If you try short intervals, these will be at ratings such as 22-24-26, and your power will need to be in proportion, so 150-165-180 Watts.
Using pace tends to be confusing, since it is proportional to the inverse cube root of power. Power is what the erg measures directly.