The ability to hold FTP is inversely proportional to IQ.
As a younger man, I was an average pack filler Cat 3 weighing 76-79Kg. I can't get below 83 Kg anymore, I am into my 60's. I made 318-325 watts (average) on a Powertap G3 hub over the course of some Strava races in duration of 20-40 minutes and made 282 watts (NP) over a 2:40 C race, which I won. I was 102 Kg at the time, I am almost 6'4''. If I was 70 Kg, that would impressive power. Anyway, I am quite certain I could have held 300 watts for an hour but my longest balls to the wall effort was only about 45 minutes.