I second the heart rate over cadence method. The correct cadence is what is most comfortable for you. I'm not a high cadence person and I can still keep up with the 20-22 mph riders. Today's ride was a 39 mile ride with a max speed of 22.99 (riding with a group) and my average cadence was 61. Amanda Coker, who is breaking the woman's world record for most number of miles in a year, rides 230+ miles a day and averages 20+ mph and her cadence is less than mine. I know because I have ridden with her on several occasions.

I also have serious doubts about slow cadence causing knee issues. Since getting back into riding, six years ago, I have always had a slow cadence and have put in over 41,000 miles with max cadences not much higher than 80 rpm. At 70 years of age, my knees are doing just fine. Amanda is on day 117 of her quest and already has over 26,000 miles and her knees are doing just fine.

Learn to use the momentum of the bike along with your gears and spin at what is comfortable for you. You'll do just fine. A sign written over the counter of a bike shop I once visited, read, "The more you ride the better you get and the better you get, the more you ride." Follow it and trust me! It works. Just try different combinations of gearing and cadence until you find the one that works best for you and stick with it.