terrymorse 

It is interesting to see what Coggan says about VO2max training (level 5) and mitochondrial function, which I assume is based on data. From his "more checks are better" chart at each level:

Expected physiological/performance adaptations resulting from training at levels 1-7

1. no checks
2. two checks
3. three checks
4. four checks
5. two checks
6. one check
7. no check

So, according to Coggan, mitochondrial production is equally sub-optimal at zones 2 (endurance) and 5 (VO2max), and if your goal is to increase mitochondria, you should be doing more levels 3 and 4.