Having sex more than 3 times in one night. Each time lasting more than an hour.
Last night me and my boyfriend had marathon sex, we didn't go to sleep until 6 this morning!
Sex that continues for hours per session, punctuated by winded intermissions wherein the parties involved sweatily seek to restore the normal oxygen-carbon dioxide mix in their blood; Sex that burns as many calories as a marathon.
Our marathon sex session came to an end when both of us were simply too tired to continue.