Thanks for all the replies,everyone. Here's an update now that the ride is done.
I ended up not using a backpack. Temperature and humidity was high yesterday right from the start, so that was a good call as a backpack would have added to the insulation on my back. I was able to stow all of with I needed, including an extra action camera, USB power bank and cable for my Garmin, and various bars and PB&J bagel sandwiches using a combination of a large saddle bag, a top-tube bag, the aforementioned fanny pack, and my jersey pockets. We had three planned scheduled stops spaced about 80-90 km apart, but we made quite a few unscheduled stops for fluids. That was absolutely crucial with the heat.
This is one of those bucket-list type rides. Some of you may be familiar with it. It's the called The Hairshirt. Here's the link:
https://tbn.ca/hairshirt
Though the event was official cancelled this year due to COVID19, lots of riders like ourselves still went ahead and rode the route at the scheduled date and time.
Edit:
We finished with a total time of 11 hours and 18 minutes, averaging 31+ kph with about an hour of total stopped time.