Sierra, good evening I hope you are well and having a great night.  With regard to your question, consider the following.  Fartlek was developed by the Swedish coach Gösta Holmer back in the 1930’s. It was originally used by runners, but a lot of other endurance sports have adapted it as a part of their training.   The only problem is that it is impossible to reproduce a good Fartlek training session because the amount of aerobic and anaerobic training is based on how spontaneous you are and therefore the outcome will vary from time to time.

We hope this helps.