I have really enjoyed listening and reading about Jack Daniel's principles on running. Here is a short video from one of his lectures regarding Principles of Training.
He breaks down training stress into four fundamentals:
1) the amount of work we are doing,
2) the intensity level we are performing it at,
3) the amount of rest in between work bouts and
4) the frequency in which we do them. When our training plateaus, any small adjustments in one or a combination of these principles will have a positive effect on our fitness.
Daniels, PhD,, Jack. (2013, Jan. 15). Principles of Training. Retrieved from https://www.youtube.com/watch?v=m63Cwdubuf4&feature=youtu.be