Here is an excerpt from my thesis:
The purpose of this phase is to increase your aerobic threshold to the maximum extent possible. While ascending to peak mileage you have been doing all your runs easily and slowly. For the first few weeks at peak mileage you should run slowly. Then you should gradually increase the effort of your runs to a ?strong aerobic effort?. What is a strong aerobic effort? This is the most critical and tricky part. From a science standpoint it is simple, you want to do all your runs at between 70-99% of the aerobic/anaerobic threshold. You never want to go slower than that or your progress will be delayed. You never want to cross that threshold because it will delay your overall development since scientific testing has proven aerobic exercise is 17 times more efficient than anaerobic exercise. Two or three runs in a row that cross into the anaerobic range and you soon find you are breaking down and have to run much slower for a few day to recover and your forward progress is interrupted. Simple concept, but difficult to put to practice. Runners who master the art of training aerobically come the closest to reaching their full potential. Most high school runners either go too slow or too fast or a mixture of both. Most college runners don?t do high enough mileage and what they do, they run too fast. Most recreational road runners run too slow. Lydiard says to run aerobically, you should be ?pleasantly tired? at the end of each run and always know you could have gone faster. Successful experienced runners have learned to ?go by feel?. They know when they have increased the pace to the point where they have crossed the line and start to feel uncomfortable and they back off. Less experienced runners and runners who want to try to be more precise use heart monitors. Heart monitors are a good training aid but keep in mind they are not as accurate as one would assume. The reason is there are errors in determining resting heart rate, there are errors in determining maximum heat rate, the formula for determining the aerobic range is based on average people and you may not be average. All these errors can make your calculated ?target range? to be somewhat inaccurate from your true target range. Monitors are an aid but not an end all solution.
Lydiard says it is better to err on the slow side, but keep in mind it will take much longer to achieve the same results. So it is a tricky game to run as fast as possible without ever going anaerobic.
Once you master running aerobically, you want to keep the same level of effort throughout the base building phase. Consequently, your training paces will gradually quicken on average each week. It won?t be straight line improvement but it will be significant over time and the beauty of Lydiard?s program is the faster pace will be achieved with no extra effort on your part. If fact running at a strong aerobic effort as long as you don?t cross the threshold, becomes more and more enjoyable and addicting as you go onward and there is no danger of mental burnout. You will find your times dropping significantly over the weeks but don?t get caught up in the times too much such that they effect your level of effort. It is more important to concentrate on the level of effort and let the times take care of themselves. Don?t push into the anaerobic zone just because you think you should be running a particular pace. Lydiard says inevitably there will be some times during the base building phase in which you feel tired and/or sore. He says to slow down for a couple of days but don?t cut back on distance and never take a day off. After just a couple of slow days you will find your gains will be consolidated and within a week you will feel a new surge of freshness and improvement.