You want to develop a smooth, round pedal stroke. In my Spinning classes, I coach pedal stroke as a pushing down on the front of the stroke, dragging the bottom (like scraping something off of your shoe) and then pulling up and over the top. there are exercises to develop each part of the stroke separately, but I prefer to work on the whole stroke at one time. As you get better, and as your leg muscles get stronger, the transitions between the parts of the stroke disappear. The advantage of the Spinning bike is the fixed gear, which naturally encourages a smooth stroke anyway. I would concentrate on the stroke, increasing your endurance with that stroke and maintaining a higher cadence, as opposed to worrying about your speed. That will come naturally as a result of the improved pedal technique. Also, using a higher cadence in a slightly lower gear will ease the burden on your joints (especially your knees).