I've been doing short z2 rides on the trainer a few times a week, and using running and rowing for intensity, and throwing in a bit of strength training as well. It seems to be working surprisingly well for how low my total volume has been. My z2 HR/power is pretty close to what it was when I was riding a lot more, and I did an impromptu FTPish effort a little bit ago that was maybe 20w off of what I did at camp last year. I've also been trying something called PQQ, if anyone has heard of that. Its supposed to promote mitochondrial biogenesis via pgc-1alpha (same as long endurance rides if I understand correctly) which if it does seems like it would be really helpful for endurance. I found one study that didn't show any improvement but I'm not fully convinced. Wondering if maybe time frame or testing mode could be tweaked. I've been taking 20mg (when I remember) an hour before workouts for about 2 months now. CoQ10 also looks interesting