Rowing is 75- 80% legs, easy. You drive with your legs until they are nearly fully extended, then you start your lean with your back and pull your arms to your chest. Check with this- http://www.concept2.com/us/training/technique.asp
Either you're doing it wrong, you're in better shape than you think and aren't feeling the burn due to good muscle stamina, or you're such a short sh-- that the rowing movements are lost on you.
Seriously though, 30 mins on a concept2 should make even the most conditioned athlete cry for mama. Next time keep an eye on the stroke output on the monitor and work on bringing it down to under 1:50. Graphic here- http://www.concept2.com/us/service/monitors/pm2/level1.asp . Th