I have been working with Eric's programs for about a year now, and no matter what heart rate or speed I run at, I still tend to keep to 21-23 right footfalls per 15 seconds, even in WUs and CDs. What you have to do is shorten your stride to go slower, and lengthen it to go faster. As Eric has said before, the hardest thing to do in the beginning is to run slowly at a regular cadence. I found that the easiest way for that cadence to be engrained into my mind is to run without music, as unless you find music with that particular bpm (or footfalls per minute), you are going to be speeding up and slowing down to the music. For some reason, no matter what I listen to now while running, I always keep the same cadence; but I spent 8 weeks without music to start with.