I don't worry about this. The reason I've read classics is that I assumed they must be worth reading. Where I've started a classic and found myself still not enjoying it after 50 pages, I've just quit. Tastes differ.
The longer the book, the more tentatively I approach it. If "Don Quixote" were only 200 pages long, I'm sure I'd have read it by now. As it is, I have to admit it's not top of my "must read" list.
I'm in my 40s now, so I have had lots of time to get things read. I don't feel inadequate about books I haven't read. I read a lot of non-fiction, and would rather have the broader view that this provides than be able to tick off every single book in the "100 all-time classics" list.
All that said... To some extent it is also about the destination. I think the goal of "getting something done" is common to much enjoyment... For example, I do a bit of computer programming as a hobby, but if I actually ask "Am I really happy?" while I'm doing it, I realise that the satisfaction comes from contemplating the completed task rather than the activity itself.
There are a mere handful of books that I found so compelling I could keep reading them for hours on end without wanting to do something else. ("Tess of the D'Urbevilles" and "The Unbearable Lightness of Being" are two of them.) For the most part, I find reading quite hard work, but satisfying work nonetheless.