What do I Need to know about Indoor Rowing?

Ok this “workout” kind of evolved. There was no plan. I ran through the trails near my house and partway through a 15km run ended up at the track and ended up doing a 5000m…initial km at the track was 5:06 and then decided to keep going for another one and gradually round them down ending the last one at 4:30 pace. Final 5km was 24:24. Actually quite dissappointing given all my run mileage but the weighing scale does not lie. Then got on the erg and just started playing around and doing some intervals and cruising and I realized at the 3400m point if I did a moderate hard 1600m I would break 21:30 for 5000m, so I picked it up a bit and was rowing at 1:58 per 500m speed. So I ended that 5000m rowing basically doing a mix of intervals and cruising 3 min faster than going “pretty hard” for the 5000m run. I think I can get the 5000m run down to 22:30 with a proper race effort and the 5000m row, 20 min should be doable (its holding 210W, but I have never rowed moderately hard for that long). Generally I do 400m, 1000m and 1600m hard rowing or 20 strokes super hard and 20 strokes super easy. I never go moderately hard for any longer than 6+ minutes.