Its not turbo but indoor exercise --> real exercise related.
Pre lockdown I was paddling 3-4 times a week. Typical training session would be 10km overall, (2+2 warmup/cooldown) + 6km of intervals pushing hard against other people. One longer steady on weekends, about 15km (about 1.5hrs). Plus 20miles a day commuting on the bike.
Not been doing much paddling at all since lockdown, but have a routine of bodyweight exercises and using a 35kg weight bag.
When I do get out in the boat, my initial speed is really good. I can warm up and sustain 12kph for a couple of km (erm, comparison to cycling, that is probably like doing +40kph). After 30 min I'm struggling to sustain pace.
So I think that the lesson to learn from that is that there is no substitute for hours in the saddle/seat. Short (sub-hour) efforts can build power, but stamina (ability to sustain speed) only comes from hours of exercise.