henry copeland:

Here's How Muscle Memory Works | Outside Online

The most significant adaptation to endurance training is an increase in the amount of mitochondria in your cells. When you stop training, the amount of mitochondria declines again. But the extra nuclei, which stick around, contain the genetic information that controls the formation of new mitochondria. As a result, the Temple researchers demonstrated that when you start training again, your cells are already primed to ramp up mitochondria production more rapidly if you’ve been fit before.
- www.outsideonline.com