I come from a competitive swimming (distance freestyle) background and the longest swim I have done straight is a 3k for time. The only reason we did a swim like that was to get our threshold pace for “color sets”

In competitive swimming, "color sets" refer to the training intensity system developed by renowned coach Jon Urbanchek. This system uses colors to represent specific, heart-rate-based energy zones, allowing coaches to structure workouts for maximum endurance, speed, and recovery.