Someone I know wanted to start writing her book before she'd worked out what would happen at the end. She had the first half of the story, but nothing in the second half. Should she start writing? She seemed confident that by the time she got to the middle she'd be able to muddle her way through. She'd find out what happened in the rest of the story as she went along, she said.
I know some people do this. Some of them are even pretty successful. But it's a terrible idea. If you do this, no matter how successful you are, you're not writing the best version of your story.
Here's what I said in my reply to that writer:
The best-case senario is that you manage to muddle your way through and find where the story should go, but that version won't be the best the story can be. That'll be the version of the story that the reader could've worked out for themselves, or the version of the story they expect. You need to be a step ahead. You want to know you've thought through every possibility and found the BEST story possible.
So... try accelerating the process. Go through the first half of the story as if you're writing it, but without having to write it. Sketch it out on cards or speak it aloud, following your notes. Then see if you can feel your way through one version of the second half of the story. Record it and make notes on what you said later. You'll come across the sticking points. Write them down.
Write down some questions for yourself. Then do branching possibilities. Either this or that. Do some what-if questions. Find what-if's that surprise you. Even if they don't make sense with the first half of the story as you have it - at this point you can still go back and change your plan for the first half.
Improvising or feeling your way through will mean much more work in the long run. Or it'll mean you settling for a version of your story that's OK, but not great. An obvious story. Challenge yourself. Do better. Explore every corner of your story and wring from it the full potential.
Get a strong visual sense of a single moment at the end of your story. Work backwards from that to work out what you need in the story to make that moment as powerful as it can be.
Remember, stories are change. So: what do you want to be different at the end of your story form the beginning? Different for the characters... different for the world you've created... different for your readers. Make lists in each category. Make those differences more extreme. Make them more meaningful. That's how you work out where you've got to and it might change your thoughts on where you have to start. That's how you build your story.
Your story is you telling the world how you think life is. So spend every waking moment thinking about what you want to tell the world about life. It's fine if you want to improvise it. But don't subject the reader to your improvisations, your muddling through. Give them the best, the most powerful version of your story.