In reply to Philip:
I have an arthritic back and plumped for an elliptical trainer which I use every other night. Not great for core but it is great cardio and superb for hill fitness as it has a high resistance setting.
For core work I just do general calisthenics, pull ups, fingerboard etc or even sometimes go climbing.
Fwiw, I find rowing machined really boring but I can jump on the cross trainer for an hour and watch a film. (I tend to watch horror as you don't really need to engage brain and they are more fun with an elevated heart rate)