I had a Concept2 Model C from a long time ago; I set it up in the basement in front of an old TV and would row 40-50 minutes most nights while watching ESPN. I started slowly, but gradually built up, making each session just a little longer or harder than the last. I would make sure that I was breaking a sweat ten minutes or so into the workout and kept it there for the remainder.
Rowing was the best way to start because it was non-impact and I could go as easy or hard as I wanted with every stroke. The Concept2 Forum was also a tremendous source of support.
Total weight loss: 75 lbs.