Maybe you should try a really low gear, say mid-60s and work on spinning as fast as you can for as long as you can and keep doing this. You will eventually improve your cardio fitness by doing this and it will be easy on your knees.
You could also try a higher gear and cycle up some steepish hills. This should improve your leg strength. But it might damage your knees, depends how strong they are
Maybe you should try a really low gear, say mid-60s and work on spinning as fast as you can for as long as you can and keep doing this. You will eventually improve your cardio fitness by doing this and it will be easy on your knees.
You could also try a higher gear and cycle up some steepish hills. This should improve your leg strength. But it might damage your knees, depends how strong they are