@NagariinDon't worry about it overly much. I had planned over half my post beforehand. Besides, most of my first posts are my largest anyway because I can embellish on backstory.
But nothing's really stopping you from editing your post to add stuff before walking through the portal so long as nothing contradicts what's already written. The opening is left vague in details so we can be free to write how our characters get there, what's going on in their heads, how they started their day, and more.
Edit: I'm also waiting on an answer from Yuuta regarding post order, if there will BE one.