You should make sure your critical power (rFTP) is up to date and accurate. For road running events marathon distance or shorter, your power target is easy to calculate as a percentage of CP.
When doing an Ironman, your power target will be lower as you already spent energy during the swim and ride. There are a few options:
1. During your training cycle, do a critical power test after a long bike ride. That will allow you to get an adjusted CP value that you can then use for your race (i.e. 89% of CP for the marathon distance).
2. Various coaches have a method to set a power target. Below image is from Chris Hague:
3. Some more helpful links:
Racing webinar with Coach Chris Hague: https://www.youtube.com/watch?v=mgAPFE6xKq4
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