Honestly I think for a 500M, most people can go insanely fast for about 300 meters and then get very fatigued both muscularly and cardiovascularly and sort of ***** out. Outside of dying from a heart issue on the spot, nothing is gonna happen if you (the general you but you too you big stud) just focus on one row at a time, balls out, that last 200. Literally, just focus and continue rowing with all your might as fast as you can, as strong as you can using all your muscles. Those last 30 seconds will be up so quickly, all that "agony" will feel like it was so silly to even worry about, afterward.