I've never quite got Fartleks.  Thr training plan was for a 50 minute one today.  It seems to me that a Fartlek is unstructured interval training and it is the unstructured bit that I find difficult.  If I have understood it right you run in bursts when you choose to and for as long as you choose.  I find it difficult to motivate myself to stretch my effort if I don't have to.

I much prefer structured intervals eg 5 minutes hard x 4, 2 minute recoveries.  Having that specific end point keeps me going beyond my comfort zone but if I was doing fartlek I would probably only go hard for a shorter time.

Anyway had a go today for 50 minutes using a circuit of the country park that had some hills and did the hard efforts up them.  It actually worked Ok.  Still not convinced but don't feel so negative about Fartleks.