Just checked, my first 600 was in Illinois in June. 7 finishers 3 DNF. Daytime high was 100F, full sun. I distinctly recall looking forward to the sun setting so it would stop scorching me. Even when it was halfway below the horizon, it was still killing me. Brutal is the word I used to describe it, and it's a wonder I rode another brevet. OP mentions August, which is typically hotter than June.

Pack your sunscreen and lip balm - don't forget the lips. 4x water bottles isn't a bad idea.

August in the Midwest can also mean thunderstorms, hail, lightning. We have tools like epicrideweather and randoplan today, but I still prepare for more extreme conditions than forecast. A 600 is long enough that forecasts change. And note, 5% chance or rain does not mean it won't rain.

Don't mean to be Debbie downer. Do it, it'll be epic.