Select music from your iTunes library, set a targeted pace to be your goal, and hit the start button to start running! It's that simple!
Once you start running your pace will control your music, you will know if you are running at your targeted pace if your music is playing. If your music is paused then you are running slower than your targeted pace and in order to get the music playing again you will have to run faster to reach your goal of running at the pace you selected before you began the run. This way you will always know if you are running on pace if your music is playing. :)
TL;DR - Run faster than your goal and you get to listen to your music, run slower than your goal and your music will pause until you run faster than your goal.
Note: Continued use of GPS running in the background can dramatically decrease battery life.