marathon runner vs marathoner

marathon runner and marathoner both are nouns.

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

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

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

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

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