at first, i thought, how do you not have time for EC's? but then i saw that you were taking some really hard classes, AND that you're about a year ahead in school. with that said, i would suggest taking some time off to get some shadowing, because without that, you have a slim chance of getting into any med school. they really want you to have exposure in a clinical setting, because they need to know that you're comfortable in that setting. not to mention, doing that may help you decide whether or not you want to go the med school route. if not for anything else, just give yourself a break to enjoy life. doesn't even have to be a full departure from school, just take a lighter load.
with a 3.9 GPA, i imagine you're comfortable with science and you can write pretty well from your WI classes, so both MCAT or the GRE shouldn't be too challenging for you. i'd recommend finding some prep book though, just to have some practice with the format and such.
finally, for your interviews, i mean, a few (2-3) shouldn't distract you from your 18 hour course load.