I would have thought that sessions on a rowing machine (with good technique) would help with the cardio work, and will certainly get the legs in order. Correct technique will help strengthen the lower back too. Important to sit upright when pulling the stroke and keep the back straight so that you can open the body up to allow efficient breathing. I guess you could just do spin sessions, but a bit of cross training never hurts, and if it is something different to basically just sitting on a bike again it helps to keep the interest levels up. Since the absolute root of the power in a strong rowing stroke comes from the legs, it pretty well follows that it can benefit cycling. It works the other way around too, as cycling is a big part of the GB rowing squad's land training regime. Ever seen a top rower with small legs?? Me neither!