In NZ and UK English it is "a marathon runner" - I initially thought "marathoner" was Konglish!
We would say "marathon runner" here in the US, too. "Marathoner" definitely sounds like a made-up word
Note that in Korea, a marathon can refer to a race of any distance, not only the official distance of 42 km.
I'm a marathoner in Korean, but I'm only a half marathoner in English, hahaha.
How did you figure that out by the way?