IPA: /mɛrˈæθənɪŋ/
KK: /mɛrˈæθənɪŋ/
The activity of running a long-distance race that is typically 26.2 miles (42.195 kilometers) long.
She has been training for marathoning for several months to prepare for the big race.
Marathoning is derived from 'Marathon' (from the Greek 'Marathon', referring to the site of the Battle of Marathon in 490 BC) and the suffix '-ing' (indicating the action or process of). The term refers to the action of running a marathon, which is a long-distance race of 26.2 miles.
Think of the historical site 'Marathon' where a significant battle took place, and remember that 'marathoning' means the action of running a long race, just like the famous run from Marathon to Athens.
No commonly confused words.