What's the point of underlining like that? I mean you highlight something to make it stand out, but when everything stands out nothing does. If you really must highlight such an obscene amount of text, maybe draw a vertical line next to the paragraphs instead, and add a note in the margin about why you marked it. If you need to find the passage again you could also put a postit on the page. But underlining with a pen? That's just poor style.