You have a triple whammy, actually, and maybe a quadruple, in that you have far more to explain than most applicants. The GMAT is the biggest, actually, and the one you can control. You absolutely have to get that score up to be competitive at top 20 programs. I think you need 700+ to have any shot at the schools you've mentioned.
If you can get the score up, then you next need to canvas the programs to see where you will get screened out for already having an MBA. Some schools (like Sloan) will still openly consider you, but many others will basically rule you out. I've written about this before, but as I said then, I don't have an exhaustive list of such schools because we work with very few clients who possess MBAs and its a case by case basis.
Beyond the GMAT and the degree issue is your age. I don't think you are dead in the water the way many experts would call it, but I do think it poses more questions. I have written about age extensively in these forums, so in the interest of time, I encourage you to check for those. Maybe just search the word "older" in the Ask Paul thread and I'm sure you will find them!
Finally, there is the notion of diversifying your profile. I think you pre-MBA (well, this MBA) transition to marketing helps on that front, as would perfect applications. We can certainly help you, but I think you need to reach this stage first.
Hope this helps. Sorry to be so blunt, but I wanted to make sure to respond and give you some guidance.
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