My impression: this is quite atmospheric and nice to listen to. That whistle in the beginning in particular is neat. To me it adds a bit of mysterious quality to this piece. Like the quiet numbness after being blind sighted by something that caused an unexpected shock.
The only issue I have here is this piece seems to meander a bit, stylistically; the setup is great but it drifts in direction throughout the song and as a result it feels a little unfocused IMO.
Well done overall, though. :)
As for doing this all in one go: It’s impressive you were able to do this. That said, I don’t know your workflow or process, but I’d like to share with you that I personally find works for me to help with when I write music is to take at least a few minutes of break time throughout the day while working on a project to distract from the song, in addition to taking breaks between days. That way I can come back later with a fresher mind.
I hope that helps you as well. :)