This is a Swedish term that literally means “speed play”. It involves a number of bursts of effort over a variety of distances with a variable recovery. Originally the length of effort was based on the terrain, for example, pushing harder every time you came to a climb, no matter how long it was. But you can adapt it for your needs. This is a great way of introducing some faster work into your training. We don’t use many Fartlek sessions in our plans as they can be a bit unstructured, but they are certainly a great training tool.