I used to do that whenever there were shadows (i.e. speed up in the shadows) and go slower in the sunshine, to make my runs 'more interesting' - I also speed up and slow down on the bike in the gym - and also do it on the cross-trainer.
I tend to call it 'High intensity interval training' but I have to say the work 'fartlek' sounds like an alternative I can go with. But I think people will wonder what I'm doing if I say, I'm 'fartlekking' today. (Unless they are Swedish, in which case they'll probably correct my grammar)...!
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