Yeah, congratulations. I did my first Sprint distance 4 years ago for my 40th. I have being doing the occasional Tri ever since, my first Olympic distance was last year.
This year I focused more on cycling and completed the London to Paris in 24 hours (inc. ferry time) last weekend. This is why I never post in the thread 'did you clean your car this weekend', I never had the time !
I highly recommend doing an open water swim, I still go to a lake every week, at least from May to September.
As you say you can eat like a big and not have to worry about weight gain, that's pretty much why I do it ;-)