Country in which one did marathon? (4)

I believe the answer is:

iran

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'country' is the definition.
(Iran is a country)

'one did marathon?' is the wordplay.
'one' becomes 'i' (Roman numeral).
'did marathon?' becomes 'ran' (I can't explain this - if you can you should give a lot more credence to this answer).
'i'+'ran'='IRAN'

'in which' acts as a link.

Can you help me to learn more?

(Other definitions for iran that I've seen before include "Country lying to south of Caspian sea" , "Farsi-speaking country" , "Nation" , "Previously known as Persia" , "Middle East country" .)

I've seen this clue in The Telegraph and The Sun.

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