I'm not a visual artist, but I write music (guitar and piano). It's a hobby, nothing "serious". I find that when I make it a point to play every day for at least 30 minutes, my playing, my creating and my overall happiness with my output grows exponentially. If I have a period where I can't play regularly, like busy at work or with general family stuff, my playing/creating/happiness with output goes in the toilet. I suppose creativity is like a muscle that needs exercise.
I recall once listening to a Jerry Garcia album with my brother (who also plays guitar), an acoustic album, don't recall which one. My brother was remarking at what a great guitarist Garcia is (was), but I said yeah but what else does he have to do all day? He can sit and play and write for days on end if he wants. That kind of repetition makes for great skills.
I'm not discounting the 'born with it' types, I do believe they exist and it comes very natural to them. But what if they weren't channeled into their art? What if Paul McCartney never joined the Beatles and became a teacher? Of Da Vinci went into the clergy? Would their greatness have still come through?