I just finished NaNo (I won), and now I'm reading back over what I wrote in November's frenzy, and feel like more than half my words happen before the book really starts. I'm not sure how that happened because I planned everything out really carefully before I started writing. Is it just me being stupid? Or did I accidentally write a prequel?
Not The Beginning
Dear Not The Beginning,
Firstly, congratulations on winning NaNo! Whatever ends up happening with them, you wrote 50,000 words in 30 days and you should be proud of yourself.
As for your question, without reading your words, it's difficult for me to know if you're right, but my instinct is always that you should trust your intuition. If you feel like the story actually starts more than halfway through what you've written, you're probably right. I had exactly the same issue doing NaNo a couple of years back. I realized when I hit the 28K mark, that the story really started at that point, not way back where I started it.
But all is not lost. Some of that stuff you wrote at the start is likely to be able to be used somewhere later in the book. In a flashback maybe, or single scenes may slot in elsewhere.
But even if you can't use any of it, those words helped you find the story and your characters' voices. They brought you into the story and gave you backstory for the events that happen in the book. Whatever you write from here on will be informed by those words you may never actually use.
And don't forget, those words helped you figure out where your story actually needs to begin. A strong opening is crucial, and needs to be as close to the inciting incident as you can get without confusing the reader, or leaving them unsure who the protagonists are.
So don't feel like you wasted time in November. You didn't. Just keep writing until the end, knowing everything you now know about the people and the world you created.