road runner vs marathon runner

road runner and marathon runner both are nouns.

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

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

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

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

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