Good to go in fast and hard for the full distance (6)
I believe the answer is:
length
'the full distance' is the definition.
(something's total length)
'good to go in fast and hard' is the wordplay.
'good' becomes 'g' (abbreviation).
'to go in' means one lot of letters goes inside another.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'and' says to put letters next to each other.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'g' inserted inside 'lent' is 'lengt'.
'lengt'+'h'='LENGTH'
'for' is the link.
(Other definitions for length that I've seen before include "Object's longest dimension" , "Distance from one end of an object to the other" , "Duration, extent" , "End-to-end dimension" , "stretch" .)