2000m is the standard for benchmarking yourself. Legs/back/arms are three distinct movements - when you are done pushing with your legs and they are fully extended you should still be upright, then lean back, then arms. The arms should just be dead weight used as a hook for the oar up until the last part of the drive. Engaging them throughout will tire you faster. Reverse the order on the way back to the catch.

If you're feeling particularly motivated - bring a bucket ;)