In a fartlek session you speed up and slow down, blending continuous training with interval training. They are best done not paying too much attention to pace but instead feel.
One of the main benefits of a fartlek session is it works both the anaerobic and aerobic systems.
The session can be done virtually anywhere: trails, roads, parks, flat or hilly terrain.
The session can be structured, for example: run at hard effort for one minute, then easy effort for two minutes, then hard effort for three minutes etc.
Or it can be ran as freeform as you like, for example: run hard to that house in the distance, jog easy to the tree, then run hard up that hill in the distance, and so on.
If you decide to try a structured fartlek session, the shorter intervals of a minute and less should be ran a bit harder than your 5K effort, helping to improve your speed. While longer intervals for up to five minutes should be ran between marathon and 10K effort, allowing you to work on your race pace.
First warm up for 15 minutes of easy running.
Then try 3 sets of:
5 minutes at marathon race pace.
4 minutes easy jogging.
3 minutes at 10K race pace followed
1 minute easy jogging.
1 minute at 5K race pace.
30 second easy jogging.
30 seconds at 3K race pace.
In between sets jog easy for 5 minutes.
Finish with 15 minutes of easy running to cool down
An unstructured fartlek should be exactly that, unstructured, but it’s a good idea to roughly know the amount of time you will spend mixing up the intervals, for example 30-45 minutes and know where you plan to finish the session.
Once your ready, warm up with 15 minutes of easy running and then start picking out objects in the distance, such as a tree, house or a pole and run to it by feel. The aim is to get a feel for race effort while being aware of how you’re feeling as the session progresses.
Once you reach the object you picked out, jog easy until you are back to a relaxed conversational pace and feel recovered. Then pick out your next object and repeat the process until you reach your destination.