Don’t just go through the motions on your run. A fartlek session forces you to stay engaged and make active decisions about your running pace. You won’t just maintain a steady pace throughout the entire workout; you’ll be switching it up as you go, making different intervals based on what you’re trying to accomplish.
For example, if you’re running on a looping trail, you might run your race pace from telephone pole to telephone pole and then slow up. A fartlek forces you to make active decisions mid-run in order to adjust your pace and challenge yourself.