I used TBR for the Sept 2 MCAT. I will present both a short and long answer.
SHORT answer: Don't worry about it. You aren't even 3 weeks into your review for next year's MCAT. I expect you to score low now, and steadily climb as you progress. Wait for FL 1/2/3 to gauge your progress. To quote the 76ers, "Trust the process."
LONG answer:
I find that the passages are generally on-par with the difficulty of the MCAT passages. Some are harder, some are easier. If you set the overall difficulty of the MCAT at a baseline of 100, I would give TBR roughly a 105. Yes, slightly higher difficulty, but it is within reason to me.
Personally, I didn't care what my score was on the passages, especially early on. Why? Because at the beginning of your content review, you simply aren't going to score 75+% unless you were very strong to begin with. And by that, I mean you took the AAMC Sample Exam and got like 70+% I would expect that as time goes on, and you get farther along your review, you will eventually start to see your percentages rise, because as you do passages, error logs, detailed question and answer breakdowns/analysis, etc your mastery of the material will be better cemented. Even then, if you scores remained in the 50%, I would not be discouraged. 50% is respectable. 25% is not.
I would also immediately log off of FB, Twitter, IG, SnapChat, WeChat, Viber, Line, Kik, SDN, and any other social media (yes, I consider SDN social media).
I would make "error logs" of all my practice passages. I hope you are doing detailed review of your passages, and not just merely checking them right and wrong. If all you're doing is scoring them, you have completely wasted the potential of your practice passages. See "General Guidelines for Reviewing" in the following thread. Actually read all of his posts, and everything else covered in the magnificent stickied strategy thread.
Breaking Down the MCAT: A 3 Month MCAT Study Schedule
My first "diagnostic" would be AAMC FL 1, which I would take after having covered roughly 75% of the concepts in each sub-section (GChem, OChem, Physics, Bio, Biochem, Psych, Socio). That is the first time I would really take a look at what I have done so far and see if I can still make my target score by my expected exam date (within 10-15pts of my target score, depending on days left). This also the point where I start adding in Next Step/Altius FLs every few days. Then after I complete my content review, aka 100% coverage, I would take FL 3 to see where I am at. At that point, I would look at my error logs to determine where my content weaknesses are at (anything I consistently am getting wrong on FLs/passages/EK1001) to determine where the most utility can be gained from my studying time. I would also gauge again if I am close to my target (~5-10pts away) and if I can get to my target in the time remaining. If there was doubt, I would log into SDN and ask for advice. I would schedule FL2 exactly 1 week before test day, and perform it in as close to exam conditions as possible (same day, eat breakfast, drive to exam center and home, put on headphones and earplugs, start at 8, take proper breaks). If you're not scoring within a few points of your target score, that is when I would come back onto SDN and ask for last second advice.
I really don't care what I score on my practice passages or non-AAMC FLs. Why? Because they aren't written by AAMC. They are, at best, close approximations of the real thing. So how could you expect to accurately gauge your prospective score? You can't. You're just in it for the practice. Mark your progress by the AAMC FLs, not the TBR passages. If you need more review questions, hunt for the old EK1001 circa mid-2000s. They are great sources of discrete-type practice questions.