For me, writing definitely has some muscle memory to it at the thought level. It is not that I am necessarily repeating the idea in the same words, but there is a pattern to my style of writing that makes it mine and the more I write, the more "mine" it becomes.
It definitely does happen at the gym too, though age starts to play a factor in there also - which is going to be my issue when my membership starts in August.
If we are able to automate the mundane where we don't have opportunity or much need for improvement, we can free up some time for higher level change. But as you said, use it or lose it and if not "practicing" well, it is going to degrade and fall into bad habits repeatedly, until a new memory is formed that is both hard to break, and poor.