on the plus side for you, your GPA tends to matter less when you have more years of work experience. also, your story makes sense (your work supported marketing and now you want to move into that).
you should start studying for the GMAT ASAP. don't underestimate it - it's a hard test, but it can be overcome. if you start studying now and put in a few good months, you should be ready to take it with enough time to re-take it if necessary.
i dont know what the average gmat for the schools youre targeting is, but i'd say if you hit the mid 600s you should be in a good position. obviously a high gmat alone won't get you in, but it gets your foot in the door at least.
will you be applying to full time programs only? or are you also considering part time? given your profile, and the fact that you want to switch careers, i'd guess that the full time route would be the best. you'd lose on income for those 2 years, but the summer internship you would get as a full-time student would make it easier to switch careers after graduation