I'd have someone check your rowing form - I can't really imagine how rowing could be too easy. If you're applying your legs as you should, and trying hard, it's as difficult as you want it to be
33 strokes per minute. set to 10 resistance on concept2. proper form. go for 30 minutes.
I'm unconvinced this is an easy thing for an unfit individual. I'm fairly sure an unfit individual would only be capable of 2 minutes... at best.