long-distance runner vs marathoner

long-distance runner and marathoner both are nouns.

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

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

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

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

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