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When I download the RW'sGarmin-Ready marathon schedule it gives me a .wkt file. My computer can not recognise this - what format should I save the file so that I can upload the schedule to my Garmin 405?
Your computer doesn't need to recognise it, your Garmin (or training centre) does.
As Johnny J says - fartlek is not really structured, just a run, usually off road, where you vary things up - some v fast sprint bits, some a bit longer at a fast pace, some hill reps & some slow/jog. I believe the term can be translated "speed play".
Intervals are more structured, and can either be all the same distance at the same speed with a recovery, or could also be a "pyramid session" where the intervals get longer (or faster) and then back down to the original one.