Originally Posted by
Fiery
Relax and stop pushing. You want the pedals to feel light under your feet, offering little resistance. Ride slowly in your lightest gear, say 5mph, and notice how it feels - that's how much you should be pushing when going at a comfortable cruising speed, only your legs should be spinning faster. Don't focus on spinning though, focus on having that light pedaling feeling while keeping the same speed. After you get used to the feeling of spinning and staying on top of your gear, you'll be able to keep the high cadence even when pushing hard.
I agree with this. Along with a cycling computer, this will get you where you want to go. Cadence and form are very important, especially if you have knee issues, so keep it relatively light and work over time on increasing your cadence, which will also eventually increase your speed. Use the downhill sections to practice higher cadence with light loading, and still get the benefit of decent pace because of the descent.