Starting with an olympic distance base, a half requires 4-5 weeks of build and 10days of taper. As an ex-swimmer with a history of running injuries, my longest training run was only 1:20 (about 10mi). The pace isn't really much slower than your olympic distance pace. Maybe 1-2mi/hr slower on the bike.
Olympic distance stuff gives you all the speed you need and you only need to put in a few weeks of longer aerobic stuff to have a solid half. These weeks should feel pretty easy, just long. They are for testing out nutrition. (~200-300 calories/hr depending on weight).
Plus, you have a lifetime as an athlete standing behind you. All those double days for college polo will help you out now. PM me if you want more info.