1. Time spent at or close to your VO2max has been FINALLY proven to be the MAIN factor for performance gains. Now, go hard! 

  2. BUT make sure you are not JUST going hard. It seems that you also need a high 4mmol threshold and some higher levels of training volume (read: fat oxidation) to make this work.

  3. SHORTER intervals (30s on / 15s off) might be better for you, if you are struggling to get a high threshold % during your intervals.