My initial experience was very similar to what you're reporting. Rather than jump right into a full exam, I prepared by taking a few 10 question "Learning" quizzes. I consistently scored in the 60-70% range, and made a bargain with myself I'd continue studying, re-watching certain PrepCast lessons and taking "Timed" quizzes until I could consistently hit 80% or higher. For me, this was helpful in a couple ways; most importantly I now knew what the exam was going to look and feel like, which helped focus my thinking while studying new content. For instance, while learning something new (say Qualitative Risk Analysis), I started asking myself questions about the topic and thinking about how one could create questions to test this item ex. what is the intent of this step? what makes it different than Quantitative Analysis? why is this knowledge area ordered in such a way? what does it require in order to function (in other words Inputs/Outputs)?...etc.
For me, asking those questions while studying helped transfer the knowledge from memorization into learning. After only a few days of this approach I was hitting my 80%'s and decided I was ready for my first practice exam (Exam #1 in the Exam Simulator)....I got 136 correct out of the 200, otherwise known as 68%. I was discouraged, but I learned a couple very important lessons that I carried with me into my real PMP Exam:
1.) Don't let the clock intimidate you. I still had 45 minutes left to review my marked questions and that was on the first try, my time left for review got steadily better with each practice exam. Also, watching the clock makes you rush through questions, don't let that happen to you. You'll miss questions you would have gotten right had you slowed yourself down (at least I missed them).
2.) Insert a couple mental breaks. 4 hours of deep thinking is a long time. Upon review of my first practice exam, I noticed I missed quite a few questions that I really did know. After pondering, I came to the conclusion I was mentally fatigued. In subsequent practice exams, I made a point of staying aware of my focus. If I noticed I was drifting or skim reading, I stopped, stood up and stretched/took a 30 second walk and came back refreshed. It helped. This may sound obvious, but diligently taking practice exams also helped. I noticed my ability to focus grow longer and longer the more exams I took.
3) Keep track of the questions you struggle with. I kept a notebook beside me with 3 columns (Struggle, Skip, Changed). As I took each exam, I'd jot down the question number in the corresponding list. After completing each exam, I'd of course review each of the incorrect responses, but I also went back and reviewed each question I jotted down. I did this for a couple reasons, first being a correct guess, even an educated one, is still sort of wrong. You want to learn that item so you don't miss it, or something like it, in the future. Second, ever go back and change an answer and find out you had it right the first time? Happens to me all the time, common wisdom (Cornelius also says this in a PrepCast Lesson) is to trust your gut, but sometimes you can't help yourself. I kept track of each question I changed, and if the change was correct. Turns out about 75% of my changes went from right to wrong. Math is never wrong, I immediately stopped changing answers unless I was POSITIVE and my scores went up. I also stopped taking guess that I would be forced to consider changing later. If I didn't know it, I'd skip the question and come back later. Again, my scores went up.
It goes without saying the majority of the knowledge I needed to pass the PMP Exam came from the PrepCast Lessons and the PMBOK Guide, but those 3 tips I mentioned went a LONG way toward helping me improve my test exam scores. I think it was between taking test 4 and 5 that I scheduled my PMP Exam, so in this case my personal experience blends what the others in this string recommend and your proposed approach. I didn't want to schedule the exam until I had demonstrated a certain ability in the exams, but at the same time, there's always going to be a part of you wondering if you're ready. It was in this mindset I scheduled my exam. I was doing well in practice, not as well as I wanted but well nonetheless, and I found adding the pressure of the real exam helped get me over the hump (I passed first try). My advice, try not to worry so much about your first few scores. Get familiar with the layout, keep studying and learn from your mistakes. You'll be alright.