If you're comfortable with your swimming now, as in, not learning from scratch, I'd bet you can build up the endurance you'll need by December. The base you've built biking will help you on the basic endurance side, and practice, practice, practice will help you on the swimming endurance side. Masters swim twice a week will probably whip you into shape, as long as you're swimming at least the distance of the tri

I think you've got a good plan, and I'd do it.