You have some good advice and links here!

I would lower your gear ratio, and learn how to spin fast. If you are geared too high (and I think you are a bit high), you will have better top end, but your average speed is going to be slower. There is a lot of slow down and speed up in urban riding, and a lower gear is going to help.

Do you use clipless pedals? You should, as this will enable you to spin better and more importantly allow you get your power from the upstroke rather than just mashing (and stressing your knees).

If you ride clipless and can work on a smooth stroke (i.e. not mashing) you will take a lot of stress off your knees.