As much as rowing seems like an exercise in the arm, most of the strength for each stroke really comes from the leg push. It's also important to note that it's not just how many strokes a minute you can do, but about the quality of each stroke and how much space you can create for each stroke.
On the C2 rower:
Pull a sub-1:25/500-m pace for as long as possible
Rest 3 minutes
Pull a sub-1:30/500-m pace for as long as possible
Rest 3 minutes
Pull a sub-1:35/500-m pace for as long as possible
Rest 3 minutes
Pull a sub-1:30/500-m pace for as long as possible
Rest 3 minutes
Pull a sub-1:25/500-m pace for as long as possible