Yes, FARTLEKS is a Scrabble word!

Letter values in Scrabble US: F 4, A 1, R 1, T 1, L 1, E 1, K 5, S 1.

Is it a valid word?

FARTLEKS is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

a method of athletic training (especially for runners) in which strenuous effort and normal effort alternate in a continuous exercise