well I think you can do them both ways...I think where the confusion is you are doing fartleks which are HIIT by definition...bodybuilders do HIIT intervals where they do sprint recovery work...i.e.20 sprint 40 recovery...the total duration includingwarm up and cool down is 20-30 minutes...it is done this way because it is muscle sparing unlike longer duration cardio and the afterburn affects last longer.

This seems different than what you are doing for endurance training...i believe thatis where the confusion came in...same terminolgy, different sports, different methods.