Rowing (and all other forms of exercise) are definitely non-linear in progression. Most of the time the progression is sub-linear, but rarely you get the other thing too.

From 2013~2023 I would struggle to row 10km in any duration. My typical daily routine would be 2~3km. From 2023+ I can row 10km every single morning 365 days a year without any issues.

Some kind of step-wise change occurred about 2 years ago. It wasn't a gradual or linear event. I vividly recall a day where I just kept going and going without paying attention. I finally look down and there was 14km indicated on the display. At this point I figure I can do 4km less than that every day since it wasn't too bad. 80% of it seems to be psychological. The human body is incredibly adaptive, even over short time frames.

Confidence is probably the most important thing in making progress with things like weight lifting. Definitely in Olympic lifting. I've seen people go from a 135lb to a 225lb clean & jerk in one day with a good coach.