marathoner vs marathon runner

marathoner and marathon runner both are nouns.

Word	Noun	Adjective	Verb	Adverb
marathoner	Yes	No	No	No
marathon runner	Yes	No	No	No

As nouns, marathoner and marathon runner are synonyms defined as:

marathoner and marathon runner: someone who participates in long-distance races (especially in marathons)

Other synonyms of marathoner include long-distance runner, road runner.

marathoner (noun)	marathon runner (noun)
someone who participates in long-distance races (especially in marathons)	someone who participates in long-distance races (especially in marathons)