Writing a piece “without thinking” is a good sign you’ve done a lot of thinking for a long time.
Every piece I wrote used to take my entire brain, and hours of work crafting the perfect solution to the musical challenge at hand.
Today I wrote an entire arrangement for my group in around an hour (with passive planning time, driving and so forth). Admittedly this was made easier by knowing the song for my entire life, and by knowing the ensemble intimately. But part of it is that my arranging toolkit has slowly, painstakingly, become quickly accessible in more situations. I knew just the tools to reach for, how to use them, and thus got the music out of my head in no time.
There’s a feeling, at the end, that it was too easy to be of value – it only took an hour, after all! But the hour came after over 20 years of work building skills, learning what works, and developing my tools. The speed is the result of years of slow work. It felt like I did it without thinking, because I did so much thinking over the past decades.