The word Fartlek means "speed play" in Swedish. Naturally then, this classic workout originated in Sweden. It calls for runners to switch the pace from slow to fast often during a training run. The benefits of doing this is you build endurance and speed during the same workout.
The way you incorporate speed into your training is really up to you during this workout. A simple way that I use this is I will add five, one minute surges during a regular distance run. I wait until about ten minutes into the run before I start mixing them in, and try to finish all my surges before there is five minutes left of running.
The speed on each surge is dictated by feel. I try to make my faster parts at or quicker than race pace. This is how a Fartlek builds endurance or aerobic capacity, and speed or anaerobic capacity.
A more structured workout that I make my cross country runners do involves running hard for one minute, easy for the next, and then repeating this up to ten times. Some of my younger runners don't grasp the concept of running quick. They will an all out sprint right away. Once they see the older runners running fast, but in control, they understand how to run this workout much better.
Here is an example 50 minute Fartlek workout for my cross country team.
1. 15 minutes of easy running. (The warm up run) 2. 1 minute hard, one minute easy x 5. 3. 5 minutes of easy running. 4. 1 minute hard, one minute easy x 5. 5. 10 minutes of easy running. (The cool down)
Remember, incorporating "speed play" into your run is easy and really up to how you feel. You kind of run like a kid again during this workout. Run slow when you are tired, and pick it up when you feel good. If you don't think you can handle the freedom of running like this, just follow the workout my high school runners do. Now your ready to play with speed.
Still looking for more to help you run a fantastic race? For a complete workout program designed to help you run your best marathon, I recommend The 100 Day Marathon Plan. It is a wonderful resource if you are serious about improving your time, or you are training for your first marathon. Marius Bakken, the creator of this plan, is a two time Olympic Distance Runner.