Nothing... about Ironman is fast. The race is about not slowing down. Really.
Since 1997, I've seen the same thing. Someone is going to "train a new way" for Ironman, or "train only with power" (guilty myself in 2007). Save yourself the embarrassment of walking during the marathon. Don't do it.
There are two energy systems in your body.
1) Aerobic - "with air" generally considered to be 80% of max heart rate or less. For RPE, stay under 16 (out of 20). Monitoring your heart during exercise is the most effective way to train for an Ironman. (*along with power on the bike - we'll get to that) Changes in your heart rate correspond with changes in oxygen utilization by your lungs, breathing rate and lactate accumulation in your muscles. Most athletes cannot determine when lactate begins to accumulate and the exact point that they "go anaerobic".
The body prefers to burn fat for fuel when you workout. This is most efficient at aerobic heart rates. Aerobic exercise improves oxygen utilization and uptake and is the chief predictor of endurance performance. Aerobic exercise is energizing to the body.
If you are training for an Ironman 70.3 or full Ironman race - the BULK of your work needs to be HERE. 70-78% effort and average heart rate to your workout sessions.
NOTE: If you are doing an Ironman or 70.3 - your average heart rate will be 70-78% average for the entire day. Promise.
2) Anaerobic - also known as A.T.. This is a physiological state where your body starts to accumulate lactate (lactic acid) in your muscles. Go hard enough and you actually stiffen up. This pace may not be "all out" but it is very strong. Usually between 85-95% effort. Once you are in this zone, you've got about 30-45 minutes before you will need to slow dramatically.
What is the zone between 78% and 85%?
I refer to this zone as the "grey zone". How long you can stay there is part genetic. Part training. Part years of experience (read: training). If you are training long... avoid it (in general).
Some general rules for training:
1) ALWAYS wear a heart rate monitor. Even if you are training with power. You WILL want to know what your heart is doing for the entire workout session as well as when you do intervals.
2) Breathing- most individuals do not breathe with the efficiency necessary for maximum athletic performance. Put your hand on your belly button. Take a breath. Did your hand move? If not, you are shallow breathing. The diaphragm is a flat, parachute-shaped muscle under the lungs. It works like a bellows. As you inhale, this muscle contracts drawing air into the lungs.
After my first Ironman the muscles that hurt most - the intercostal muscles that run between the ribs and just under my ribs - the diaphragm. Basically... I hurt from BREATHING. The other muscles didn't hurt by comparison.
Diaphragmatic breathing is a great way to get your heart rate down in a set.
The body will follow the mind. If the mind is in control, it will not allow breathing to become irregular. Focus on deep breathing (diaphragmatically). This is a very powerful, performance-enhancing technique.
It's a common error for exercise instructors to tell you to "tighten the core" or "pull in your abs tight" - even during sprinting. This will hinder your most optimal breathing.
**READER'S NOTE: If an instructor EVER tells you to do that a) ignore them, b) after the class - ask them for the physiological reason why. Then listen as they make up a reason.
A real life example of keeping the heart rate down.
One of the students in my computrainer class has been training at 155w (watts) FTP. She tested at 167w, but couldn't hold that level of power in sets. I lowered her FTP and told her not to worry about it and "trust me". For the majority of every session, I had her FTP (in the CompuTrainer MultiRider program) down at 155w where she was able to keep her heart rate down around 75-78%. During the "power" portions of the class each week, I'd bump her up to her tested at FTP. This was about 15-20 minutes of work. How'd she test? Well... she opened up her FTP test averaging 185w for the first 15 minutes and then negative SPLIT the session, bringing her FTP avg for the session UP to 193w.
Ironman and Ironman 70.3 isn't about what happens in the first two hours, but what happens in hours 3+.
If you are a first time Ironman racer - wear a heart rate monitor and obey it like it is God. Practice with this leading up to the race. Pretty soon, you'll know what heart rate you are at when your breathing changes. This assumes that you can heart your breathing and you aren't training with an iPod on.
Talk to your coach or an experienced athlete to help you plan how to get stronger both aerobically and anaerobically. This will help your next race.