Entry 2.5, Why the system works.

Perhaps a long post for a simple explanation.  Skip to the end if you get bored.

Your body uses a chemical called ATP to make your muscles contract (as well as a whole bunch of other stuff).  At any given moment, your body only has enough ATP for about 8 seconds of effort, so you're constantly replenishing it*1.  While exercising, most ATP production comes from the breakdown of sugars.  The most efficient way to do this is to use oxygen in a process called aerobic respiration.  In this process, a single sugar molecule will get you about 36 ATP molecules.  Anaerobic respiration on the other hand, will only get you 4 ATP per sugar, and will produce lactic acid, which as we all know, hurts.  A lot.  Aerobic respiration is much more efficient, but it is limited by how much oxygen you can pull in, and how fast you can get it where it needs to go.  ATP production from anaerobic respiration is practically unlimited in the short term, but is unsustainable over time due to the lactic acid buildup.  At high intensities, production of lactic acid outpaces your body's ability to neutralize it.  In the very short term, we are limited by our muscles, the force they're capable of delivering to the pedals, and the speed at which they can spin.

As road cyclists (and in particular, those of us who dislike sprinting), we very rarely use more than our body weight, so our legs are plenty 'strong' in terms of how much force they have to deliver to the pedals.  What we're really working on when we train are the first 2 mechanisms: Aerobic and Anaerobic respiration.  It's a weird way of thinking of it, but at the end of the day, the strongest cyclist is simply the one who is the most efficient at turning food into forward motion.

As long as your body has stored sugar and oxygen, Aerobic respiration is superior.   You can get more work done with a limited supply of fuel, and it's relatively painless.  Anaerobic respiration is necessary for energy needed above what aerobic respiration can handle*3.

Ok, so there are two systems, maybe I've gotten too into describing it, but the TL;DR is.  Aerobic respiration is good for sustained efforts below a certain intensity and Anaerobic respiration is necessary for high intensity efforts.

Here's the part related to training though:  Aerobic fitness takes longer to gain, and takes longer to fade than anaerobic fitness.  This is why we spend the bulk of the season on low intensity work, and why we do it first.  You are maxing out your capacity for aerobic gains before moving on to anaerobic workouts.

*1.  Molecular biologists and physiologists will say I'm oversimplifying most if not all of this, but for the sake of our purposes, I think this is fair.  Feel free to clarify if it will contribute.

*2.  Track sprinting is almost a different sport in this sense.  Sprinters are much more focused on pure force strength than any sort of endurance.