I can see advantages and disadvantages to your system. Advantages being less required mileage and quickly getting into shape with LT and 5k/LT economy sessions. However, some potential disadvantages could be inability to recover from heats in a championship due to insufficient conditioning (through periodization) as well as potential performance plateauing and potentially loss of motivation from doing the same sessions every week. Have any of your athletes showed inconsistency in training/racing (e.g. not racing at training levels or or not re-signing with you after a goal race)?