Ugh.
My brain is toast after 2-hour meetings. I can't actually imagine anything productive coming out of an all-day meeting.
I've had days where I had meetings nearly the whole day, BUT the total hours in meetings was only 6, and at least they were typically an hour a piece, so my brain had a small amount of time in between to refresh and also the various topics/content kept me a little sharper.
If I got stuck in an all-day meeting like you're talking about, they'd probably have to wheel me out on a crash-cart by the end of it.