This is precisely how I have been preparing to preach for almost a decade now. Not that my sermons in any way compare to Augustine's. But the description of the process preparing them is quite similar.
Furthermore, I would break down my sermons into one of two types, similar to a description Deferrari offers in his essay St. Augustine's Method of Composing and Delivering Sermons.
Extempore--Those given after some previous meditation on the subject, but with no extensive preparation.
Strictly extempore--Given unexpectedly and without any preparation of any kind.
Most of the sermons I give are of the first variety. Perhaps one or two per month are of the second.
"Augustine notes that working from a prepared or memorized text hindered the ability to keep one's rapport with the audience... he says that he prepared with prayer. He had, of course, worked on some texts long and hard in his study."
As part of my dissertation, I have a chapter where I examine how to develop this extempore approach in terms of the neurology and formative aspects of it, and will provide a link to that essay (or perhaps post it as an e-book) at some later date. No method for preaching is built in a day, and there are many ways to preach. I do not offer this as prescription for everyone. But the above gives about as concise a sense of how I have learned to preach without notes as I can offer.