The passages are the correct length and difficulty level. Many TOEFL preparation books have passages that are too short and too easy. However the questions themselves seem to be a little more difficult that the ones found on official tests. These are closer to questions found standardized tests used for entering Graduate levels of university. The website is targeting students at the Graduate level, and so this difficulty level makes sense.
It isn’t a bad idea to practice with materials that are a little more difficult, but if you are not getting 50% correct, the materials are too hard and you are probably guessing too much.