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:

https://alancouzens.com/blog/run_power_pacing.html

https://www.velopress.com/how-to-use-run-power-data-to-pace-ironman-triathlon-marathons/

https://the5krunner.com/2018/10/22/him-running-power-with-stryd-hm-too/

Racing webinar with Coach Chris Hague: https://www.youtube.com/watch?v=mgAPFE6xKq4