The reason is simple: Entropy.
Entropy was the featured Word of the Day a few days ago on the wonderful homepage I use, REFDESK (do go check it out if you are unfamiliar).
REFDESK linked to The Free Dictionary, where we read:
In popular, nontechnical use, entropy is regarded as a measure of the chaos or randomness of a system. In thermodynamics, it is the measure of a system's energy that is unavailable for work or of the degree of a system's disorder.
See, if we consider me to be a system, and if I have an excess of entropy, it ties up too much of my energy, rendering it unavailable for running purposes. That's why I have draggy runs sometimes.
So I guess as part of my warm-up, I must go through some entropy-purging routine. Next step is to figure out just what that would consist of!