The Ice Cream Marathon is an event that is held in the Land of Ooo. It is mentioned in the episode "The Pods." The objective of the Ice Cream Marathon is to see who can eat the most ice cream, possibly while running 26 miles, or possibly just eating a 26 mile trail of it.
Jake was determined to participate in this marathon and win it, but was delayed when he and Finn stumbled upon a Gnome Knight guarding a chest of magic beans. When Jake is trying to convince Finn to abandon the job of defending the beans, he states that the Ice Cream Marathon is "twenty six miles of ice cream." It is never clarified whether Jake was able to attend the event, though at the end of the episode it seemed as if he was going directly to it.