The human body never ceases to amaze me. In harsh conditions, the body learns to adapt to the experiences it has received and use that experience to 'perfect' the body. Every day, these experience changes, and every day, the body adapts in a different way. One day, I run 20 miles and go cycling and stair climbing. The next day, I just lay around and not do too much other than what I would usually do on a Sunday. Both days, I accumulated experience. Both days, my body used that experience to better itself. On Saturday, I gathered experience regarding the how much the body could handle when running 20 miles without anything other than water, and efficiently work to push forward. The next day, I was able to accumulate experience of doing... 'nothing' and the body used that time to take a break and recover.
If I had run day after day, my body would not have had that time to recover... but experience would be piled on to the body to help it grow and adapt to the best possible shape because of whatever I chose to do. When I did yesterday's run, I ran without that much food in my system, making my body work not just from the stomach, but also from the fat stored, burning it as fuel (as well as burning unneeded muscle). My body just gets ready for the every day things that happen, and gets used to whatever I do, which is why I like doing what I scheduled or practiced.
If I run 20 miles a day, over time, my body too would begin to get used to it, and be able to do the 20 miles without too much exertion. Some runners use weighted vests to confuse the body to run with more weight but the same speed. With that extra weight, the runner's body takes in the experience and information, and works hard to be able to adapt to that. Due to the adaption goal the body would attempt to build muscles that would be able to compete with everything that is going on, and when used, it would be able to go faster, and the runner would be able to push harder, giving the body yet another thing to experience, getting used to it and going forward.
Now, just as important as the positive adaptation of the human body, there is also a negative side. When one becomes a couch potato, the body adapts and begins to store energy in fat because the body isn't using energy. Whatever happens, be it good or bad, the adaptation happens. In order to change how the body adapts, the job of the individual is to create that experience that would push the body to adapt the way they want it to adapt.