Those mile runs -- makes you supple! (9)
I believe the answer is:
lithesome
'supple' is the definition.
('lithesome' can be a synonym of 'supple')
'those mile runs' is the wordplay.
'runs' indicates an anagram.
'those'+'mile'='thosemile'
'thosemile' with letters rearranged gives 'LITHESOME'.
'makes you' acts as a link.
(Other definitions for lithesome that I've seen before include "slim and supple" , "Flexible" , "Graceful" , "flexibility" , "Willowy" .)