2015 USATF Club Cross Country Champs, photo by Justin Britton - Fartlek is speed play. It is a fun way to develop speed and build endurance. The great runners in Sweden in the 1920s and 1930s were the first to use it, on soft trails. The beauty of Fartlek is that it can be spontaneous. Short speed runs broken up by easy runs, long speed runs broken up by similar easy runs. Most elite middle distance runners use Fartlek.
Northern Arizona Eite, XC 2015, photo by Justin Britton - End of year is a great time to access what you did int 2016, and begin to make plans for 2017. That is why I always put three recovery weeks into the end of the year. You still will be fit if you run a midnight run or an early season indoor track race.