I think it's true that in English extensive reading (tadoku), I should take books which are not difficult to read. For the primary purpose of extensive reading would be to digest real usage of English as much as you can.

So again I've stopped reading an English book on the way, a book which I recently bought. It was because I found it rather difficult for me to read for now. But it's okay. I've got other books to read.

In this purposeful reading, I should always choose books according to whether I find them interesting and easy-to-read so that I can maximize the quantity of pages read in a day.

Inasmuch as it's being done under this particular purpose, this way of reading books would be reasonable. The time you can spend to read is not infinite and books readable which there are in the world is so numerous that obviously you can't read them all.