You should assume a couple of things as you start your GMAT journey:
1. Relearning / rebuilding your quant skills takes a lot of focus, energy and time (away from work, family, friends, life in general).
2. The GMAT ultimately isn't a Math or English test -- it's a test of executive reasoning skills (which speaks to your potential as a future business decision maker). Ultimately, they don't care about your math prowess -- the GMAT uses math as one way to test your reasoning skills.
With that in mind, you want every minute of your time spent to be precisely directed to a great GMAT score.
I wouldn't suggest spending more than a bit of time on general math resources like Khan Academy. (don't get me wrong it's a great organization and resource). You'll spend way to much time and energy building quant skills you don't need. Like Scott said, most GMAT resources assume you are rebuilding your core quant skills (but only the ones related to the GMAT).
I'd strongly suggests getting some support in the form of a coach for both diagnostics and keeping you on track. That doesn't mean your prep can't be primarily self study. You just need an expert who can maximize the ROI on the limited amount of time, energy and effort you can spend on this.