I went through a phase where I was constantly fatigued by intensity (group rides and chasing KOMs). What cured it was a block of 10 weeks of very low intensity riding (base miles) - 10+ hours a week of riding at talking pace (70% of HRmax). After that block, my FTP was up 20 watts and I was crushing Strava PRs on the group ride.
@doctorhammond I’d start with this on VO2max intervals, it was my earlier point about elite intervals. I prefer simple intervals, starting with something like 4x2-min all-out on each interval, then progressing to 4x3-min, etc. Based on what you said, 8 minutes (4x2-min) or 12 minutes (4x3-min) is plenty of stimulus. If you are on sustained power build and just did Fish -2, then Kaiser +2 is coming up and that is 3 sets of 3x3-min for a total of 27 minutes. See the difference? Kaiser +2 is nine 3-min intervals, while a classic starting point is four 3-min intervals.
And FWIW I’ve also determined that TR SS base does not properly prepare me for vo2max work. What does appear to prepare me better - doing a longer base (4-6 months) with up to 80% of my time doing aerobic endurance / zone2 workouts. During that time I’m also keeping my top-end fresh, but the focus is on building a “stronger base” with all that aerobic endurance work.