Try out this playlist for your next run. It's set so that you have a warm-up song, 4 miles worth of songs at a 10 minute pace (almost 40 minutes - just borrow some time from the cooldown song to complete your run) and a cooldown song.
Going out for a longer run? You need a playlist that will take you through those miles. As long as your pace is at or under you should have just the right amount of music to take you through a one-song warm-up, a run and a one-song cooldown.