I don't remember the wording of the preface, but I recall it arguing that the sheer size of the book could be regarded both as a cultural experience as well as a mental exercise.
The author of the preface argued that when (or perhaps if) you finished the book you would feel like you've just run a marathon. You would be highly fit, reading wise. Nothing would phase you.
This was probably compounded in my teenage head by the fact that the only quote I've heard from the author beforehand was:
"I consume libraries. I wear out spines and ROM-drives. I do things like get in a taxi and say, 'The library, and step on it.'"
It sounded like a complete opposite of myself at the time, but I did want to learn that ability. And I liked that preface a lot. It kept me going at it month after month on busy train rides, with the promise of some transcendent reward at the end.
There are famous footnotes in that book spanning several pages in tiny print. The amount of mental effort it took to read the thing was extraordinary for someone like me at the time. I often had to backtrack several pages to perform the context switch from a mini-novel footnote back to the main storyline.
This was many years ago now, and I have slipped far behind on my reading fitness. But I still carry with me the realization that I can't expect to be fit if I don't train hard.
Now, ADD is a whole nother beast. Sicknesses often require medical attention and this should not be smoothed over. Still, I think there's an interesting discussion to be had regarding reading fitness in the era of information. Maybe this is a widely discussed thing and I'm just not hearing it, but it seems obvious that we slip further toward obesity without training. Surely something similar happens with our ability to focus without training?