For athletes looking to get a solid base seasons in, but don't have the time, nor the days... while volume will always be king during base season, adding some intensity will allow you to maintain your aerobic maximum and allow you to start hard training at a higher point than your peers.
Feel free to add in additional volume during the week... if you wish to add a 5th day, I recommend moving the sweet spot intervals to Friday (no endurance afterwards), and then adding endurance to Saturday and Sunday. You may also do a shorter endurance ride on friday (<90mins or less) on Friday, and the intervals on Saturday.
Vo2 maintenance is performed every two weeks. If you are older than 35years old, I would recommend Vo2 intervals every week instead of every other week. Simply replace the threshold intervals with VO2.