0 km. No stress.
After an exhausting Monday Erling has done some calculations for the headwind effect on paddling. 3 m/s is like pulling up a weight of half a kg, 5 m/s more like 1.5 kg, that’s ok. 7 m/s is more like pulling up a 3 kg weight, that starts to be a struggle, and 10 m/s is like pulling up a weight of 6 kg. And good luck with that.