「マラソン」 is a Japanese word. It should not matter what the word "marathon" means in the original language from which Japanese borrowed the word.
Likewise, 「[手紙]{てがみ}」, in Japanese, means a "letter", but 「[手紙]{shou zhi}」, in Chinese, means "toilet paper". There are no problems with that, however, because those are two different languages.
Well, so much for preliminaries. The word 「マラソン」 can be used to refer to any running event if the distance is "long" by someone's subjective judgement. The "official" マラソン is, of course, 42.195 km.
When I was a kid, which was before we had the word 「ジョギング」, many people around me called "running around the neighborhood for 10-20 minutes for fun and/or health" マラソン. We did not really have another word for it.
マラソン does not even have to involve running, believe it or not. 「ヒアリングマラソン」 is a method of learning a foreign language where you keep listening to recordings for hours at a time. 「[禁煙]{きんえん}マラソン」 is for those trying to quit smoking. You try to break your own record in how many days you can go without smoking so that your brand-new reward pack that you give to yourself for making a new record tastes incredibly great.