I had a couple of slow periods - both when a particular book was heavier going than I expected.
I'm not too worried about the book total/page count challenges though. I like keeping track, but I'm not heart-broken to miss such a challenge. In the two years I've set a goal, I haven't reached it. Oh well.
I'm more interested in all the sub-challenges I set myself. Next year I'm focusing on paperbacks that I have in my bookshelf and haven't read yet (new), classics (again), indie read/reviews (again), short stories (new) and award winners (new).
These challenges are more to focus my reading. For next year I've also started putting together a suggested reading list for some of the challenges to make it easier to bounce from book to book.
At the moment, these challenges suit a purpose for me. I don't find them a burden - it's actually fun to watch the progress of the challenges over the year. It's a game I play with myself in the time between finishing one book and picking up another.