马拉松 is a loanword from English 'marathon'. It literally refers to a long-distance running race of 42.195 kilometers, but is frequently used figuratively to describe any long, grueling activity or event that requires sustained effort — e.g., 马拉松会议 (a marathon meeting) or 马拉松谈判 (a marathon negotiation). Unlike transparent loanwords like 咖啡, its figurative use is common in both spoken and written Chinese, so you need to recognize both senses.