Hello, and welcome!
I think that you're going about things the right way: work on fundamentals first, and try the more complex problems later on. The Q and V scores that you will ultimately obtain are inextricably linked to your understanding of the basic mechanics. As such, your most important task is to prepare yourself adequately on the fundamentals. The
Manhattan GMAT (
MGMAT) books are always a good place to start: The Foundations of GMAT Math should put you on-course for the quantitative aspect of it. I am unfamiliar with any basic Verbal books, although I am certain that this forum has plenty of people who have required those, too, so don't be afraid to ask!
Personally, I think that a month or two should be sufficient in terms of getting a decent grip on the basic elements. From thereon out, it's all about honing individual skills (for example: you know that if I have 3 green marbles and 4 blue ones, I have a 4/7 chance of picking a blue marble, but do you the likelihood of picking a blue, followed by a green, followed by a blue, in that order, without replacement?) Obviously, my example is pretty easy, but it shows the thought progression that will need to take place on various elements that the GMAT tests.
After your initial 1-2 month period, you'll need to spend a good 2-4 months doing many practice problems and reading more material (such as the rest of the
MGMAT books, or some similar prep material).
If you take anything away from this post, let it be this: there are people who do thousands of questions, it's true. Heck - I'm one of them! But that is no indicator of success. Much more important is the amount of time that you spend on the practice questions that you do. Always ask yourself the following:
1. Why did I get this wrong? Am I unclear on the concepts behind the question, did I not see the shortcut because of a lack of practice, or was it a silly mistake due to exhaustion?
2. Why did I get this right? Was it a guess or did I know what I was doing the whole way?
Practice questions from every subject tested (for math, for instance, you will have things like Number Theory, Inequalities, Plane Geometry, Coordinate Geometry, Statistics, Probability, Permutations and Combinations etc.). You can be a world class algebraist, but if you can't draw a coordinate plane, you'll do terribly! In any case - spending 10 minutes on one question can be incredibly valuable if those 10 minutes stop you from making an error on test day. Similarly, doing 5 questions in 10 minutes will teach you nothing if you've forgotten what you did and how.
Finally - we have some fantastic contributors on these forums (Bunuel comes to mind) who are extraordinarily good at solving problems and answering questions with clear explanations. If you're stuck on a question, type it into Google, which will direct you to a solution posted on this forum about 95% of the time.
A Score of 600, or even 700 or 750 is never out of reach. It's a matter of practice, timing, psychological know-how and a smidgen of luck. There are people who have scored abysmally on their first attempt, and have come back to rock the exam. Take your time, get into the correct mindset, and you'll have no problem. Good luck!