Google "Polarized training" and "Stephen Seiler" who is the most popular proponent of it.

You'll find abundant material from reputable people in the usual places like the ncbi.nlm.nih.gov, the wattage google group and slowtwitch.com

Polarized training has greater impact on key endurance variables than threshold, high intensity, or high volume training

Polarized thread on slowtwitch with contributions from seiler himself

and plenty of anecdotal reports.

It even works better for people training only 5 hours a week (6.4 average, 1.4 standard deviation):
Six weeks of a polarized training-intensity distribution leads to greater physiological and performance adaptations than a threshold model in trained cyclists