What is the origin of fartlek?

Fartlek “a training technique consisting of bursts of intense effort” is a direct borrowing from Swedish and comprises the elements fart “speed” and lek “play.” Fart is cognate with the English verb fare “to go, travel,” as found in the nouns warfare, wayfarer, and welfare, from a Proto-Indo-European root per- “to pass over.” Other derivatives from this root include port, portable, and portal (via Latin porta “door, gate” and portāre “to carry”) and pore and emporium (via Ancient Greek póros “passage”). Lek may be related to English lark “a carefree adventure; to have fun” but has a far more popular relative: LEGO, the name of a brand of interlocking plastic bricks, from Danish leg godt “to play well.” Fartlek was first recorded in English in the 1950s.