long-distance runner vs marathon runner

long-distance runner and marathon runner both are nouns.

Word	Noun	Adjective	Verb	Adverb
long-distance runner	Yes	No	No	No
marathon runner	Yes	No	No	No

As nouns, long-distance runner and marathon runner are synonyms defined as:

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

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

long-distance 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)