Cycling Fitness / Training Advice

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

im planning a slightly lower ratio for my new bike. Im too used to high gears already, I have pretty powerful legs and can push hard & slow with out a problem.

Spinning sounds like it would be good for me, I cant wait till im riding fixed again tbh.

But other ideas are good too.