Not sure exactly where you are on the gulf coast, but I live in pancake flat Houston and have done IMCdA, IM Lake Placid, Syracuse 70.3, Oceanside 70.3, and Chatt 70.3; all with lots of climbing. Unfortunately, it's not easy for me to get hill work in as it usually means driving over an hour away from Houston every Saturday to get to areas with hills. None of the hills I did where as big as anything I did in any of those races, but they were short and punchy and surprisingly got me ready. I've been told that you can also simulate hills when riding flatland by sitting up and shifting into a much higher gear and doing intervals. I have friends that have gotten ready for hill races that way.

My other suggestion, don't let your ego get in the way and change your gearing. I used a compact crank (50-34) with an 11-28 cassette and never had any issues. I'm a really high spinner and the 34-28 let me keep a high spin. My thought process is that is always better to have a lower gear and not need it, then to need it and not have it.

As I mentioned, I did the 70.3 in chatt last year, and I thought the hills weren't bad at all. The rollers are perfectly spaced so if you stay aggressive on the downhills, you will carry a lot of momentum up the next one. Even my sister (who is a very weak cyclist) exceeded her expectations by gearing properly and staying aggressive on the downhill portions.

Good Luck!