Our Tuesday night session this week was Fartlek - the aim of this session is to mess around with speed because the word Fartlek LITERALLY means Speed Play in Swedish. Mess around with your speeds. 5 mins warm up, main session as long as it takes to get the Reps done, if you are using lampposts, cars as reference stay out for around 35 mins and then have a 5 min cool down easy run or a brisk walk whichever feels best.