Hi @ozmadman
I use intervals.icu and the only way to make a significant increase to your fitness score is longer duration rides. I have added a screenshot of my chart for the last 6 months and the 2 big spikes are due to a 3 day, 310mile sportive in June (3x 6hr days all around 86% intensity) and then a 6 day holiday at the end of September (6 consecutive days between 3 and 5 hours, 75 - 80% intensity).
Between these spikes, I barely kept my fatigue above my fitness, and it was simply down to a reduction in volume.
Since the start of October, my fitness score has dropped from 82 to 64, as I have started to focus on indoor sessions again. The only ride that lifted my score in this time was a high intensity outdoor ride of over 3hrs.
I have just completed a couple of weeks of MAP focused workouts including downward spiral, revolver and 14VC. I am in no doubt these workouts are continuing to improve my power on the bike, but at best, and despite their intensity, these shorter sessions are barely maintaining my intervals. icu fitness score (which I would expect to continue to drop down to the low to mid 50s through the winter).
It would be worth searching the forum for articles / posts on chronic training load (CTL) and listening to the knowledge podcast for a more detailed explanation.