The following data is from a 33 year old male and is used to illustrate the "bonk" as recorded by a heart rate monitor. The information is provided as an example only. The data does not represent a comparison standard for other athletes. Individual athletes must determine there own heart rate "ranges" .
Contrast this with the following information from the same individual 7 weeks earlier. Data is from the marathon only. Note the even heart rate during the entire run, with a slight increase at the end. The average heart rate is greater (10 beats per minute) and the overall time is faster. During the run, the individual was able to negative split the marathon.
Keeping all of these things in mind and using the heart rate monitor as just one of many guides can help you stick to your race strategy. Develop a plan that incorporates pace, feeding, and hydration so that you can survive the challenge of an Ironman.