I don't buy into FTP in my training or racing at all but each to their own. I did early in my Ironman career and FTP prediction for your race power is fraught with danger. Based on the predictions my FTP is too high for what I could withstand for an IM. You are saying it is your first IM with power so I can assume you have raced IMs before without power? I would be looking at the good races you had and any heart rate data you have and then be mixing that with perceived effort. If you believe you went too hard in the past back it of slightly and vice versa as you feel on the day. After a race with power you will get an idea your power but even then I have a window I like to be in and go to the upper and lower end of that based on feel for that race. It has taken me many seasons of IMs to work out what is a good constant power for me to put out a solid bike and back it up with running legs.
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