8k is 5 miles and is not considered a marathon. The traditional distance used for a marathon 26 miles 385 yds or 42.2 km. Typically the only other race with a defined standard distance, that uses the word marathon, is a half-marathon run over 13.1 miles, although there are extreme or ultra marathons can be run over hundreds or thousands of kilometres.
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I'm guessing the k stands for kilometers, and 8 kilometers is real close to 5 miles.