Do I need a secretary?

Sounds like whatever you do, you'll need help with all of that custard!

Only you know how the story is supposed to really go, a secretary could only make an informed guess and for a first draft, it seems like it would be a little over the top.

I would say get a pack of post its, write down a scene or chapter per note and stick them on a while side by side then read through the files with bits of novel and stick them in columns under the post it to which grouping they belong.
Once every bit of story belongs somewhere on the timeline, work out in which order in their grouping they go.

I can't comment on how efficient a secretary would be for a writer, but the ones in my office are amazingly good at keeping the architects organized... as architects and writers are both the creative type and people in our trades can be somewhat flighty, (I know I am) it might benefit you if you're willing to pay.

Here's something I just developed to help visualize mine: Get a Word doc, orient it Landscape and draw an arrow from bottom left to top right corner. That is the linear progression of your story.

Now make yourself a bunch of ovals or rectangles and arrange them all along the arrow. In the one to the far bottom left write the first scene or major occurrence in your story. In the top right one write the finale or conclusion then start filling the other shapes.

After you have a the linear aspects of your story then you may find you have some sub-plots. Place them some distance from the arrow and draw lines to show how and where they connect with the main plot.

You don't need a secretary, you need Maxwell Perkins.

But, failing him, you're just going to have to collect all the bits and put them together yourself. Once you've done that, you might hire a typist with good copyediting skills to produce a finished manuscript. From there you can go to anyone from free beta readers and writing groups to a professional free-lance editor (good ones being very far from free.)

But only Max Perkins could make sense out of something you can't make sense of yourself, and bona fide psychics are even more expensive than free-lance editors.