I did the Wattkg 9 week program (400hrs) last year after finishing sweet spot base mid volume. I had started the mid volume build phase but found that I was tired all the time and I was dreading the training sessions. I noticed some interesting results. My FTP increased by about 15 watts to 270 but this was not where I noticed the real benefits. My real gains were in my 5s to 30s power which increased dramatically by 100 watts and my 1 minute power increased by 50 watts and I set PR in all times under 1 minute. For the past few years I have had serious cramps after intense rides of 2.3-4 hrs. The is where I noticed the benefit of the LSD rides. I could ride much longer and sustain a higher FTP, without any cramping at all. Again I set personal bests on long rides at high % of FTP. I could go hard on breaks or climbs, then recover quickly and then go hard again. My club mates were surprised at how much stronger I was and were very interested in my winter training program. I found that on club rides I could ride at a hard but manageable pace and they would be struggling. I would pull for extended lengths of time, and they would complain about the fast pace - a complete turnaround from my fitness the year previously (all TR plans). I consider the LSD key to what I call “raising the floor”, the ability to ride at an endurance HR pace, but at a relatively high power level, which is something LSD rides develop. On club rides when others are in riding at low tempo, I was riding at high endurance, and almost always felt I had extra in the tank. Rarely could my club mates keep up with me on the sprint at then end of a hard 3 hour ride - a big improvement from the year before. And the nice thing about this training was that I did not feel wasted all the time which I did when I was training most of my rides at sweet spot.
So this year I am started with SSbase LV, but also adding some long weekend LSD rides, and will switch to the WattKG 9 week plan after the SSbase. I should also add that after reading up on Seiler’s work, I have reduced my training FTP, because I felt it was over estimated and have paid a lot more attention to my HR and power output together. Its really about listening to your body, not just hitting the numbers. Just so you know, my training starts in Jan at about 6 hrs per week and will increase to about 10 hrs per week come April - nothing too intense.