You’ve either got very strong legs are could use more work on your aerobic capacity. 30% of body weight should be plenty to elicit a muscular endurance effect: one where you are more limited by the local muscle fatigue than by the ability of the heart to supply oxygen. This ME must be preceded by an extensive period of aerobic development during which you raise the aerobic threshold to within 10% of the anaerobic threshold. This will ensure enough aerobic capacity to support the higher intensity work. ME workouts can be effective when done in Z2 as well but if you are very fit Z3 is better. Z4 or near max efforts like you are doing will wear you down in short order so I recommend going back to aerobic work or at the very least slowing down so you do not lower, rather than raise your anaerobic threshold.