Ancient battle associated with the messenger Pheidippides (8)

I believe the answer is:

marathon

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

'ancient battle' is the definition.
'marathon' can be an answer for 'battle' (Battle of Marathon, 490 BC). I'm unsure of the 'ancient' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for marathon that I've seen before include "Long-distance running event" , "The longest foot race in sport" , "Olympic event" , "Long distance race" , "strenuous event" .)

I've seen this clue in The Times.

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