What everyone has said so far is right on. The Admissions people are typically pretty snippy with undergrads and people far off from applying to med school. Your best bet is to find a pre-med advisor to talk to, either through the school you graduated from, or the one you plan on going to if you go back for a degree.
I have been to several informational meetings for pre-med students that had speakers from the Admissions office of U of M, Wayne State, and MSU. After listening to many of these people speak on several occasions, I can give you some general advice from people that asked similiar questions at these functions:
If you've been out of school for a couple years, they want you to prove that you can handle the course load of medical school. So you should probably take a semester or two of science classes, and do well in them. It's not necessary to get a second degree, or even a masters as long as you take the pre-med requirements and do reasonably well.
You will need a strong MCAT score. If you already took it, but it's been a few years, you should probably re-take it.
Having the degree in biochem already is definately a plus. Biochem is one of the classes first years typically have a hard time with, and chem majors in general are accepted at a higher rate than bio majors because their course load is usually more difficult and this makes them stronger candidates. The microbio background will also look good as well. You didn't mention any clinical experience though. I feel I should mention that this is becoming very important and can easily make or break you. Admissions panels want students that have a history of volunteering or working in a clinical setting. The job can be as simple as pouring water for patients or as intensive as an EMT, etc. At the school I plan on applying to, you won't even be considered if you don't have some clinical exposure. A lot of schools are following this trend. So if you aren't already, it's prolly best for you to start volunteering now a couple hours a week, while you take some classes part time to brush up on stuff.
And I know you didn't mention this, but I just wanted to give you some reassurance about your age. The people I've talked to have accepted students in their 30's and 40's before. I say this because I know it's often disheartening when you realize how many classes you need and how old you will be before you can even apply. Especially when there are 18 year olds fresh out of high school sitting next to you in your organic or physics class or something! I didn't really decide until my 4th year at college that I wanted to go into medicine and in many ways I'm starting over. It's most likely going to take me 6 years just to get my B.S. 'cause I switched majors so many times. Argh! It's nice to know that there's always going to be people that are even older than you going back and doing the same thing - so don't get discouraged by the process. Anyway, I hope this helps. You should see if your university has a pre-med student association, like a pre-med chapter of AMSA. These kind of groups often have informational meetings with admissions officers, and you could use this as an opportunity to pick their brains on your particular situation.
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