What's taken up as challenge, but punishing for runners? (8)
I believe the answer is:
gauntlet
I believe this clue is a double definition.
'what's taken up as challenge' is the first definition.
(as in 'throwing down the gauntlet')
'punishing for runners?' is the second definition.
(as in 'running the gauntlet')
'but' acts as a link.
(Other definitions for gauntlet that I've seen before include "Mexican peninsula" , "Glove of armoured leather, thrown down as a challenge" , "that may be punishing for runner" , "It may be thrown down or run" , "What might represent a challenge" .)