If Deena Kastor was running 5:01 min/mile in 2004 for AT runs, Why hasn't she run 1:05 for the Half Marathon?
http://www.coolrunning.com/engine/2/2_1/the-anatomy-of-a-medal.shtml
"We both knew VO2 MAX was important, but even more so was the increase of anaerobic threshold (AT - the point at which lactic acid starts to accumulate in your muscles). Since this became an important training objective, we incorporated the AT runs, sometimes referred to as tempo runs. We started with four miles and over a period of time, increased to six, eight, and ten miles. If we were preparing for a marathon, she would run 12-13 mile AT runs. We thoroughly believed that the longer the run, the greater the stress, the greater the consequent adaptation.
A noticeable observation was made over the five-six year period of increased volume; her AT velocity increased profoundly. She went from an initial 5:24 per mile pace to 5:11 to 5:01. I would like to state that volume runs, when combined with a regular diet of AT runs, are the most important workouts for the development of the endurance component. This brought about a profound increase in her running economy.
Equipped with these two remarkable qualities (increased VO2 uptake and increased anaerobic threshold), any athlete can then embark on running and competing at the international level."