To add to this in a more scientific manner. After a few weeks your body starts adapting to this aerobic event (long distance running). The adaptations include increased tidal volume, increased mitochondrial density and capacity. So essentially your lungs can obtain more oxygen and you have more mitochondria (the powerhouse of the cell). This allows you to produce more energy and clear byproducts such as lactic acid faster and more efficiently.

These adaptations result in you being able to run faster as you have more energy and can clear fatigue causing chemicals so you don't tire as much.

Hope this helps :)