I'd probably write them like:
一日に200ページ以上を読むのは大変だ。
"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.)
一日に200ページも読むのは大変だ。
"Reading as much as 200 pages a day is difficult."
これまでこんなページ数を読んだことがなかった。
"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 も
is:
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 大変
. 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...