One question our coaches are frequently asked is, "What is the ideal heart-rate zone for the bike and and run portion of a full Ironman?" Even for someone just hoping to finish an Iron distance event this is an important question. Many people think the answer is high zone 2 or zone 3 but training and/or racing at that level would be quite tough, even for an elite athlete.
When training for an Ironman race most of our training should be done in zone 2, otherwise known as the endurance zone, a heart rate which can be sustained over a very long period of time. Why? Because in this zone, where our heart rate is about 20 to 30 beats below race-pace heart rate, or lactate threshold, the body will utilize the biggest resource we have for fuel: fat. We have a virtually unlimited supply of fat stores that if trained properly would allow us to run back to back marathons or even longer! When we cross over into zone 3, we tap into more of our glycogen stores for fuel, and the body can only operate using stored glycogen for two to three hours, tops.
More: Training Zones Explained
Zone 3 efforts should be reserved for the final miles of an Ironman 70.3 or even a marathon, and only for a well-conditioned athlete. Going into zone 3 in an Ironman, for an athlete who is racing longer than 11 hours, would be risky, setting yourself up for a tough run. I'd rather see an athlete avoid depleting their stored glycogen and keep moving in zone 2, burning as much fat as possible! Zone 2 may seem easy, but stick with it and over time you'll see an improved pace at the same heart rate.
Get the Drift
Another consideration for most us mere mortals is cardiac drift: a condition that is likely to occur when you are getting back to normal training levels or taking on a new training regime. This is where the heart rate rises into zone 3 although your effort remains in the zone 2 range, a perfectly normal occurrence, but one that should be avoided as much as possible. This could mean walking in order to keep the heart rate in zone 2. I do allow my athletes to experience this once per week when they are starting up training but I make sure that they know to back right down and stay within the set heart-rate parameters indicated in their training plan.
Once you have a solid endurance base, you shouldn't see much in the way of cardiac drift unless you are dehydrated. If you are training or racing in the heat, you may need to let your heart rate drift and instead measure your effort by rate of perceived exertion (RPE). However, only do so if you have taken into account the extra calories your body will use trying to keep cool.