If we live in a hot place for long enough, we get used to the heat. Likewise if it is cold. True, if that has limits, but the body has a great ability to adapt in order to survive.
Which is pretty much what happens when we train. We put a little bit more stress on the body for long enough and it adapts to handle the new demands. It becomes more efficient, uses the reserves of energy we have stored up (ie the life belt around the waste). The new active you is soon the norm. And so we move on from there.
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