Organising a relay run takes a bit over 50 weeks (5,4)
I believe the answer is:
lunar year
'a bit over 50 weeks' is the definition.
(a lunar year is 354 days)
'organising a relay run' is the wordplay.
'organising' is an anagram indicator.
'a'+'relay'+'run'='arelayrun'
'arelayrun' is an anagram of 'LUNAR YEAR'.
'takes' is the link.
(Other definitions for lunar year that I've seen before include "some time on the moon" , "Period of 354.3671 days" , "Period of about 354 days" , "Approximately 354 days" , "Period of twelve new moons" .)