Awwwwwwwh. It's the most liberating feeling in the world. You turn off your internal editor. and you just write.
Things come together. Ideas. Now, you'll have periods where that is difficult, and you'll find it very frustrating at times (week two onwards).
It's a completely different way of thinking. After a while, the characters help develop the story, the plot can drive itself, and you'll have moments of genius.
If you want the full experience, just write. Give yourself permission to write whatever it is you need to write! Even if it is rubbish. At my best point, I did 23000 in a day, no problem - I was flowing. It was just a great feeling - it was pouring out, and that part of the story was actually the best.
The experience will help you write what you want to write in the future. And, maybe you'll discover what you really want to write isn't what you thought you 'should' write anyway!