I didn’t have that much reading to do for my undergrad work, but the volume of reading for grad school was overwhelming. I timed how many pages I could read in a certain amount of time (e.g. 15 min) (of course, some books are more dense than others…). Then I would break my reading up by dividing total pgs to read by the amount of time I required to read them. You’re right, if you do it all your reading at once your brain rebels & turns to mush or explodes.
If you’re reading English lit, another thing you could do that might help you retain it, is after reading, look at an online site like sparknotes (just google your title) and read their analysis. It helps make sense of it all & you’ll remember more when you have to regurgitate it on a paper or test.

I’m taking two classes right now, so the workload is manageable, but I’ve found that watching the Shakespeare movie version to better understand his plays. I also like audiobooks.

When it’s over you’ll wonder how you did it, but you will!! Good luck!