If you're looking for a cardio workout, you should be aiming for a cadence of over 100 rpm in a gear that doesnt put a lot of pressure on the pedals. If you're riding clipless, you can also use your hamstrings to lift the pedals on the backstroke. When you're in the groove, it feels like your feet are floating.
Since you've already caused some injury, keep the gears low and start at 80 rpm and move up in speed- NOT in gears.