I'd probably write them like:
"Reading more than 200 pages a day is difficult."
("200 or more pages a day" but I'm not sure the difference of a single page matters here.)
"Reading as much as 200 pages a day is difficult."
"Until now I haven't read such a number of pages."
According to Chocolate's comment, it looks like
も is better than
ほど for expressing a degree here. According to Edict, the second sense of
2: about (emphasizing an upper limit), as much as, even
You could probably use
難しい／チャレンジ／大チャレンジ etc in place of
読むことは大変 could also be used instead of
読むのは（が）大変 I think.
I have no idea if these sentences are natural or not so please consider this a temporal answer...