I think as you get down below a 2 minute split (obviously the threshold is lower for experienced rowers), the power you need to generate goes up steeply, needing more strength, and in turn more oxygen. At some point you will hit a wall you can't get over, due to physical limitations or the inability to breathe in enough oxygen. I know for me, I think the lowest reading I managed to generate on the rower was something like 1:32 or 1:36/500m.... I physically couldn't pull any harder, and I could only manage that power for 5 or 6 strokes.

I'm not sure I've ever done the 20k row. I think the most I ever did was a straight hour at a steady pace. I've only ever made a real effort at either 2000m, or a gym competition which was a 1 minutes sprint.

Funnily enough, my best ever 2000m time (7:04) was the first leg of a gym "sprint triathlon" which also included a 10km cycle and a mile run on the treadmill. So in theory I could have possibly gone a bit faster, as I was supposedly saving myself for the cycle and run. But perhaps it helped me to relax and think about my technique more.

I do an unholy amount of squats. Don't be afraid of using weights, you don't have to go mad. I find that with no weight at all, I can basically do endless squats and it's a bit of a waste of time. But just 10 or 20kg on the back may even help your technique as you slow down a bit, and you can (well I can) still work on endurance and knock out 20+ squats...

One weight exercise I'm dead against (and it has no relevance to rowing) is calf raises. I used to do loads, with a bit of weight on my back. Whether it was coincidental, I had Achilles problems at that time, and once I stopped calf raises the problems went away (although I also changed trainers, and it all coincided with an increase in training and racing mileage, so I'll probably never know exactly what was going on). I now do no weights on my calfs and they are in the best shape ever (except for the strained calf I got yesterday in the xc which caused me to retire, but I'm sure that's just coincidence again...!!)