Base training (those long, lower intensity rides just at/under aerobic threshold aka ventilatory threshold 1) increases mitochondrial density. More mitochondria = more fat burning capacity. But the mitochondria are also key to lactate clearance, which has a big effect on FTP/MLSS. I first read about this in Thomas Chapple's
Base Building for Cyclists, but I'm sure it's been discussed elsewhere.
I can definitely say that long steady Z2 rides during the base season have helped increase my FTP.
Hmm. I would have thought sweet spot training would have a bigger affect, hour per hour, if only because maintaining Z2 for a long time is relatively easy, but maintaining ~95% of FTP is relatively hard for more than a few minutes...
Should you not do much too Z2 base after some point (build period?)?