You’re still running the same amount of miles
In: 37
But your body is not working equally hard.
The point of exercise is to strain your body’s resources. In the case of aerobic exercise like running, the main resource that runs out is oxygen*: your muscles are consuming oxygen as fast as your lungs can provide it, and it has to resort to some unsustainable processes to keep going. In particular, your muscle cells have to swap to [lactic acid fermentation](https://en.wikipedia.org/wiki/Lactic_acid_fermentation) instead of the usual and more efficient [Krebs cycle](https://en.wikipedia.org/wiki/Citric_acid_cycle) to generate energy. This produces by-products that have to be cleared out later, but if you continually push your muscles, they build up more and your body stays short on oxygen for longer.
That forces your body to develop higher oxygen capacity, which improves your health overall. It does so by developing your heart muscle, by producing more red blood cells, and by increasing the amount of “ready to go” energy (in the form of glycogen) stored in your muscles.
(EDIT: * Strictly speaking, *extra* oxygen. Your body won’t divert oxygen away from your brain to keep your muscles going, for example. So while there’s still oxygen in your blood, it isn’t available in sufficient amounts to your muscles during aerobic exercise.)
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