In reply to mrbird:
on a rower - warm up for 500-1000m then go full out for 20sec - check the distance you do in 20 sec
set the rower to 20 sec efforts with 20 sec rests - go for it always aiming to match the original distance in each 20 sec - repeat until you fail to match your distance 2 reps in succession at max effort - should be able to do 6 or more efforts
watch your distance and number of reps grow with time