I have been dealing with this for a while and can’t really come up with any explanations. I’m really into the whole polarized model and keeping easy rides easy, however after 1,5-2h there is a big tiredness creeping into my legs. Even though I have accumulated quite some hours in Z1-2 (in a 5 Zone model) and I tried to do 1 long ride (3-5h) per week, this does not change much. The tiredness also carries over into the next days and they seem to give me heavier legs then intense interval sessions. I keep the intensity for those rides around 70% of max HR and around 60-65% of FTP, going easier doesn’t seem to change much, sometimes it even feels better to go harder for some reason. I can’t make any sense of that, is there any physiological explanation or what could I do to improve that training?
For my background, I was racing until the U23 category then had a break for 10 years and have been training steadily again for the last 3 years, building up slowly.