This echoes what Robert Caro said he does when writing a biography. He has done all the research and then spends days trying to find what he's trying to say. During that time, he's miserable. As it says below, that's the heavy lifting. In the case of Lyndon Johnson he said what he realized, and I'm going from memory, was to the effect that Johnson wanted to help the misery of the life he knew in rural Texas where he grew up. It informed his entire career and that was the "in" for Caro.
In a book on writing non-fiction I read she says she uses mind maps on anything that's giving her trouble and waits for the "aha" as the way in.
Create the Single Sentence
Once you've settled on your topic and decide where you stand,
the next question to ask yourself is: Can I articulate my position, my
stance, my big idea, in one sentence? In Huffington's case, after
sifting through the data, she was able to distill her message into a
single sentence: Only by renewing our relationship with sleep can we
take back control of our lives.
Distilling your message into a single sentence will make your
writing flow better, and make your key points easier to arrange. Think
of the single sentence as a lighthouse guiding you through fog. If you
become overwhelmed with an abundance of data or competing themes, the
single sentence will help you stay on track.
It will help inform the choices you make regarding what
information to keep and what to set aside in your speech. Any piece of
data, story, or anecdote that doesn't jive with your single sentence
will wind up sidetracking and diluting your message.
In his book Speak Like Churchill, Stand Like Lincoln,
former political speechwriter James C. Humes writes, "Whether you are
going to a breakfast meeting with a potential investor, making a sales
talk, or delivering a product presentation, you need to first come up
with the key message you want to leave with your audience."
Let that key message be your North Star. If you can't state your
idea in a single sentence, don't give up. Keep at it. For many
speakers, this is the hardest part of their speech — and the most
critical one.
If you do the heavy mental lifting upfront, it will be much
easier to craft clear, compelling copy when you sit down to write. As
Humes notes, "Make figuring out your bottom-line purpose (your big idea)
your first priority."