Playing what you hear, not where your fingers want to go is what I struggle with, I may have to suck it up and start practicing in 12 keys. Where for you it's a few days for a tune, a project like that may take me yearsI make sure I'm playing what I hear and not stuff my fingers like to do. I'll spend all afternoon working on two or three keys. In a matter of some days I've gotten through all twelve. No printed music involved.
In the end it's like singing on the horn. Hard to think of something better to practice. It's fantastic for the ears. Interestingly enough, playing it in all keys helps me to play it that much better in any one key. Helps in so many other ways too...