Run longer and slower for endurance (that is, do 4 or 5 mile runs very slowly), and run shorter and faster for speed (the intervals someone suggested work fine, or you can get on a track and run 200m or 400m repeats with short breaks in between until you can't take it anymore). Do both, and you'll get faster. It doesn't have to be super complicated.
I'm a big believer in speed workouts. I'll never forget the one time in high school when we did our first major speed workout of the season (10 quarter mile repeats at about 1:20 -- wish I could run that fast now!) on a Tuesday or Wednesday, then on that Saturday I raced faster than I ever had by over 30 seconds, which is just an insane improvement for where I was at and how fast I was running at the time. I perceived no additional effort in doing this. My legs just moved more efficiently.