Nice to hear you'll be aiming for your first Tri this year. Sprint-distance (I assume)? As a note - most people who come into Tri's can't really swim, so I would actually focus some decent effort there (nobody has ever won a Triathlon from a blazing fast swim, but you can lose one if you give up too much time). Running is the important component - as they say, Bike for show, run for dough. Since you have a runner's background, you will do fine (assuming you don't go belly up in the swim, and are 'average' on the bike).
There are actually (very) strict rules regarding your gender for competition - it isn't nearly as simple as stating - 'Well, I'm a pre-op (or even post-op) MTF on anti-androgens, estrogen, etc...I should compete in the women's category'. For USAC and USAT, I think there is a 2-year moratorium to compete as your "new" gender (for lack of a better phrase), i.e.,you will need documented proof of HRT, with bloodwork showing appropriate hormone levels, having been on for 2 years, etc, to compete as your new gender. At the local level, this may/may not be such a big issue, especially if you are passable - some may not notice enough to raise questions. But even in big cities, Tri and cycling communities are small and close-knit, and everybody knows everybody, so issues can arise at some other level (i.e. confrontational, in-your-face). The issues really flare up if you are a podium threat.
I've also read about the male vs female endurance sports and performance. I'm still undecided about it - I've competed in a few RAAM events (ultracycling) and while the women put up some pretty crazy times, (at least in this particular event), the men put up even more crazy, ridiculous times vs distance. Maybe it's different in running, since I think there is probably more fluid biomechanics involved in running than biking (since, at least in part, a bike "locks" certain body parts and pivot points in position).
In any case, just some HRT-gender at competition info if you weren't aware. Regardless, hope you achieve your goals (sport and non-sport) in 2013!