I agree with you about the benefits of rowing. I wish I knew how to motivate myself to get out to the garage and go it alone. A group setting would be so fun! It might be time to download an audio book and designate it For Rowing Only. (Music and podcasts just haven't been enough, so far.)
The workouts we do in class aren't just "row a long time," mostly we do intervals. Sometimes we focus on distance (1600-2000 m) or sometimes shorter, more intense bouts. That might help. But yes, I don't think I'd be as motivated if I were doing it solo in a garage.