This is a very good paper that will help you out (Intervals, Thresholds, and Long Slow Distance: the Role of Intensity and Duration in Endurance Training), http://www.sportsci.org/2009/ss.htm#_Toc245522384

Here is a good video he gave on polarized training, http://www.canal-insep.fr/fr/training-periodization-deep-roo...

There is also the Phil Maffetone 180-Formula, http://philmaffetone.com/180-formula/