Fartlek Training

Fartlek is Swedish for Speedplay and was developed in 1937 by Swedish coach Gösta Holmér. Whilst I recommend you google it and learn more about it and the more advanced methods and styles this challenge (4 weeks of Fartlek training)  is going to stick to a very simple form of Fartlek training. 
It involves interval training on a treadmill, track or road, however a treadmill is preferred as you can monitor your speeds better.