The Ironman triathlon takes between nine and 11 hours to complete. No matter how fast you are, going at full speed right out of the gate will make you "bonk" before too long and you won't finish the race. Pacing is key, and a heart rate monitor is a tremendous help in terms of keeping track. The Ironman is unlike any race you've ever done before, so using your usual pacing methods won't work.
Zone 1
Most of your training -- and some of the race -- should be done in Zone 1, or between 50 and 70 percent of your maximum heart rate. The reason is because this keeps you at a pace where your body is using your fat stores for fuel, which is more likely to sustain you for the long hours required. It may seem too slow for race training, but that's what training is for -- to teach you to go faster while maintaining a lower heart rate. The Ironman is about endurance, which is the ability to maintain higher speeds with less effort.
Zone 2
About a quarter of your training time and most of the race should be done between 70 and 80 percent of your MHR. During training, you'll use this pace to do longer speed work and your easier long runs, and this will be your "base" pace for the race itself. Zone 2 is still aerobic, using your fat stores for energy -- but it is just on the border of your anaerobic threshold. Going any faster during the race will force your body to use your carb stores for fuel, and they will be depleted within a couple of hours.
Zone 3
Zone 3 is between 80 and 90 percent of your MHR, and you shouldn't use this pace in the race at all if you hope to finish. In training, however, this is the pace to shoot for during sprint intervals and other speed training. Although it doesn't translate directly into the race, it teaches your body to become more efficient at using energy at higher speeds, and it trains your muscles to perform explosively. This helps you improve your overall speed, which helps you go faster at a lower heart rate.
Event Variation
Your heart rate responds differently to cycling and swimming. When you run, your heart rate will be right on target within the zones, but you should subtract another 5 to 10 percent for bike training. For swimming, you must subtract 10 to 20 percent from your target zones -- both of these exercises are non-weight bearing, so your heart rate doesn't climb as fast even though your body is still using fuel at the same rate. Swimming is the exception to the zone guidelines because this portion of the race is generally more intense, and it only takes about 10 percent of the total time, so train in Zone 2 for better speed and efficiency.
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