I think the first one is the hardest in that book. I scored a 5 on the first and I averaged ~8-9 on the rest. I've seen other people also start out with very low scores and end up making 8 and 9s in that book. And it seems its also almost impossible to score 13+ in that book because some of the reasonings behind some answers are off the wall. I'm not sure exactly how it compares to AAMC though. I took the old AAMC 11 practice test a while ago and got a 7 and that was the first VR test I had ever seen, I wasn't even familiar on the format. However I have seen improvement and I've only missed one question in the past 3 practice passages that I have done.
As far as advice, I would keep doing a few more and see where you end up scoring, and don't worry about their explanations and logic because I think thats where they differ the most. And for improving, I think the best thing is to just do as many passages as possible. If you don't read a lot, your reading comprehension and speed will improve moderately but I think the biggest thing is that you'll start to get a good feel for the question types and what makes an answer flawed. Do this by doing passages untimed for a few weeks, and focusing on what makes the other answers wrong, rather than why the correct answer is right. This is something I didn't really grasp until I had done many many passages, mainly from TPRH verbal workbook; you have to use process of elimination on almost every question because there are many times where I question sounds right, but another one sounds better. You should NEVER answer a VR/CARS question without reading every answer choice.
Then worry about speed and strategy later. There are tricks to getting faster too, but its better/easier to make that transition if you have a feel for the passages/questions first. You can't be a great quickdraw shooting cowboy until you get good aim first lol.
