I would say that you may not be recovering enough. During the workout, and after. If you are running 6 minute pace for your base and then start doing 2 w/os a week,you need to start running slower on your regular days or youll be fried. also, it is possible that you are running too fast during base. I know I personally run only high 6's for base(14 flat 5k pr), and I feel very fresh going into the start of a season, strong as hell, but not even close to race fitness. Base should be EASY, and overdoing it will hurt your season once intervals start. Also, 80mpw of base is not equal to 80mpw with two interval sessions. that load of work is much more, and you shud build up your mileage with intervals. start with 60 mpw and 2 workouts, or maybe 70mpw and 1 workout.
just some theories.
And while every athlete is different, people are over rating how different we are. We are all human and the basic laws of the human body apply to all. The physiological benefits of intervals is fact.