That Wahoo post is absolute garbage and lies. You do not need 20-25 hours, who rides those kind of hours. Almost nobody.
To switch from cycling, Ed Whitlock ran a sub 3 hour marathon at age 73. His regular training was three 3 hour easy "shuffle" jogs per week and short runs on other days with a rest day. Probably around 12-14 hours per week max based on my reading his old forum posts. That is for the top of top AG marathoner of all time.
Almost nobody does 20+ hours per week unless they are trying to qualify for Kona in which case 1,000 hours per year is about the sweetspot for total yearly training hours in all three disciplines.
Inigo San Millan has said on innumerable podcasts that 5 rides of 90 minute duration in zone 2 is ideal. 7-8 hours per week. He then says he goes like "Hell" once in a while at the end of the end up the hill to his house. He has maintained his VO2 max and FTP for well over 10 years.
OTOH, if someone only has 3-4 hours to exercise per week, I wonder how they can spend 10+ hours per week on BF.
Rather than miles or hours, it is far more important to know how to monitor one's recovery and whether one is getting fitter. FTP is not important to me for endurance, it is nowhere near as relevant as power at VT1, not even close. FTP is the wrong metric for a tourist or randonneur and training that emphasises FTP will suboptimize your enjoyment and under develop the correct energy pathways.
Going hard in the winter with HIIT and easing up with increasing volume as the season progresses is the absolute worst thing an endurance athlete can do. Here is some data from Couzens and a quote
https://alancouzens.com/blog/optimal_periodization.html