intervals work really well on rowing ergometers, but you're really barely working your AT system with this regimen. Most of what you're getting is LT work. To get all around fitness, you want to switch this up. Try the workout you're talking about once a week, while adding a workout where you go 2 minutes as hard as possible, 1 minute off, ten times (when I was a freshman starting on the crew, this was our most effective winter workout on the machine). I would also suggest doing a 20 minute steady state piece at least once a week to build your aerobic base. If you combine these, you're going to notice good results.