Fartlek is a Swedish term meaning "speed play," and it refers to a type of training that combines continuous running with intervals of varying intensity. This method allows runners to alternate between fast and slow paces, making it a flexible and enjoyable way to improve speed and endurance. Typically, a Fartlek session can be done on any terrain, such as trails or roads, and does not require specific timing or distances for intervals. Runners can choose landmarks, like trees or lampposts, to sprint towards, followed by a recovery jog, making it adaptable to individual fitness levels.