IPA: /ˈmærəˌθɒnər/
KK: /ˈmɛrəˌθɒnər/
A person who runs in a marathon, which is a long-distance race of 26.2 miles.
She trained for months to become a marathoner and complete her first race.
Marathoner → It is formed from 'Marathon' (from the Greek 'Marathon', referring to the site of the Battle of Marathon) and '-er' (meaning a person who). A marathoner is a person who runs a marathon, which is a long-distance race of 26.2 miles.
Think of the historic site 'Marathon' where the long race originated, and remember that a marathoner is someone who runs that long distance.
No commonly confused words.