marathon runner vs long-distance runner

marathon runner and long-distance runner both are nouns.

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

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

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

Other synonyms of marathon runner include marathoner, road runner.

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