I did three years worth of translation (and interpreting) courses as part of my degree, but nothing outside of that. Your location says New York but you talking about being an ALT, so I'm not really sure where you are but any translation course won't be a complete waste of time and I'm sure you will be able to find somethin in or around where you live (or even on-line), but experience will always be preffered. If you are looking to start out in translation the best thing you can do is try and get some experience, which I know isn't easy as you need experience to get work in the first place.
Since I have been freelancing (which may only be a few years) the main prerequisite for work has been experience, although lately I have seen more listings and companies requiring N1 of the Japanese Langauge Proficiency Test, so I would suggest working towards that, if you don't already have it.
Oh yeah, and that JET programme translation course is a complete joke. A professional translator wouldn't wipe their arse with it. But if you get the chance you might as well do it, it's essentially a free qualification.