As you head out on a long run, try to maintain a steady pace for as even splits as possible. The goal is to finish your run and see your pace line on the Garmin Training Center graph be perfectly flat. Make sure to give yourself a window of acceptability. For example, the first time that I did this particular workout, I tried to keep it within a 10 second window of my goal pace. After a couple times of doing this workout, you will find it easier to make that window smaller and smaller.
Obviously, it will help not to run in an area with lots of traffic or stop lights where you would have to continually slow down, stop, etc.
For added difficulty, tray to maintain these even splits while running up and over hills. This will have you pushing up to maintain the pace, but will force you to take it easier on the way down.
This type of workout will teach you how to maintain even splits throughout a race or workout and how it feels to hold that pace for an extended period of time. Specifically with longer races, it will force you to train to run the last miles at the same pace as when you started your run.