R.P.E. in a nutshell.

RPE stands for rate of perceived exertion.  Below is the Borg RPE chart.  It's easy to use - simply choose the number from the chart that best describes your level of exertion.  This will give you a good idea of the intensity level of your activity, and you can use this information to speed up or slow down your movements to reach your target range.  

In my head, 6 is rocking back & forth in a giant rocking chair.  20 is a rabid dog chasing me down the road, just inches away from taking me down.  When you're doing your cardio workout, you don't want to be at either end of the scale for too long.  If you hang out at 6, you might as well just go watch tv.  If you stay at 20, you'll die.  (if you did it long enough!)  A good place to be is around 65%-85% of your maximal effort.  Occasionally turning it up to your 100% mark for short 20-30 second bursts will increase the effectiveness of your workout.  It's not easy to do; usually when we throw a maximal effort burst in the middle of a 65% RPE workout, our bodies are like "umm... no."  But we do it anyway.  Because that's where you get better - when your body wants to quit and you turn it up a notch.  Our brains may not like it; but I promise you, your body does!