I don't like the word EASY. You're here to challenge yourself. These workouts are not timed. The CHALLENGING Series uses shorter distance sets.
30 minute swim
MEDIUM is mediocre. The MORE CHALLENGING series is a step above the CHALLENGING series. You've done those workouts. You're looking for something more. This series uses longer distance sets than the CHALLENGING workouts with some timed aspects.
The MOST CHALLENGING series uses time to keep you moving faster. Make the time listed. You can use a clock at the pool or a stopwatch. These workouts are generally longer in time and use longer distance sets.