The Garmin sets the HR zone from what you set as your peak heart rate. It's user configurable. You can find out what your maximum heart rate is (within reason) by doing several VO2Max intervals after a good warmup. Since you don't have any hills, that means flat out for 5 minutes, as hard as you can go, against the wind if you can. Do two of them and average the peak heart rate, then set your zones against that. Knowing your HR zones will help you structure your training, as well as keep you from going out too hard in the bike leg of the tri only to suffer in the run. A 56 mile time trial is no easy effort. I've only done 25 mile TT's and I couldn't walk afterwards.