I’m doing the same Garmin program as the OP. Only 2 weeks into a 27-week program, so I know I need to give it a chance before I’ll start to notice the benefit. The first part of the program is dull for want of a better word but is supposed to improve base fitness for you to build on later. Last year I did the MTB program, and started to feel the improvement fairly quickly and saw a significant increase in VO2max.
This year in the “century” program, I’m yet to really feel any benefit, but based on last year’s experience, I’m prepared to give it a chance and trust in the process. There’s definitely more of a low intensity focus with the “century”. I find the best way to stick in zone 2 is to do it fasted, then the risk of bonking is too high if you don’t control your effort. The other option is to go for a ride with people who aren’t as fit, makes it easier to stay in Z2 without getting bored.
edit: I did the SDW in a day before I had the Garmin to dictate my training. I just trained for it by repeatedly going up and down 200-250m climbs locally. The key on the day was just to make sure to stay fuelled with solid food. And if the going starts to get tough, check your tyre pressure in case if a slow leak. To this day I don’t know how long I only had 10psi in the rear before I realised 15 miles from the end.