One who really desires more of a Marathon? (6)

I believe the answer is:

longer

I'm a little stuck... Click here to teach me more about this clue!

'more of a marathon?' is the definition.
'longer' can be an answer for 'more' (I've seen this in another clue). I'm not sure about the 'of a marathon?' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for longer that I've seen before include "Of greater length" , "Measuring greater distance end to end" , "More extended" , "Of greater duration" , "Less short" .)

I've seen this clue in The Sunday Times and The Times.

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