CUMULUS 
5 Rounds: 
On a 2:30 clock
500/400m row
Into
10 Rounds:
On a 1:00 clock
150/100m row

This workout is courtesy of @hamplan. In the first part, we want to identify and maintain a sustainable pace. Your first 500/400 should be within 3 seconds of your last. You will be able to push your intensity harder in the second set of 150/100m distances. You should aim to finish all rounds in :30 or less. I opted for ladies distances to allow for a little more rest between each, especially on the first five rounds.