I think everyone has some sort of idea of what I'm talking about when the subject of base training comes up. But when particulars are discussed, you can hear a wide range of ideas as to how to really "Do" base training and what it is supposed to achieve. Here is a definition I ran across the other day cruising through the internet:
"To develop a well balanced athlete capable of optimally
responding to the stress of competition specific training."
This is probably the most concise definition I've run across. Take a look at the rest of the post here.
What is amazing about this description of base training is it sets a criteria that is both simple and meaningful. So many people come away with the abstract concept that "Base Training" or building their "Base" is just a bunch of long, slow efforts done during the winter. And to be fair a lot of people have some success with this strategy of endless slow miles and laps. But as Jason points out, Lydiard wasn't saying run long and slow for the sake of long and slow. He was saying the goal would be to "...achieve a definite and sustained rise in the average speed at which you practice..." This in turn squares quite nicely with the advice of Mark Allen and Phil Maffetone when using the MAF (Maximum Aerobic Function) Tests in the base building period providing a framework for feedback to determine whether or not the training load and stresses from it are sufficient for effective racing and adaptation later.