Ivan -
There are several rowers on my team, and they are all very, very strong cyclists.
Since you are clearly used to training hard, I would recommend that you consider what you wish to work on by bike training.
Depending on the races you wish to compete in for rowing, I would tailor bike training to mimic those efforts. It sounds like you have been using a bike to train steady-state muscles, and I think you may find additional gains by adding more intensity intervals to your long rides.

However, if you wish to compete in bike races, I would recommend that you focus on short "burst" or "sprint" efforts as those have no perfect correlation in competitive rowing.