When you are paddling for general fitness, you are working at about 60 to 65% of your Max aerobic capacity, you are working in that aerobic capacity zone and that can be great for managing weight control and general fitness, but when it comes to entering into a 10-kilometre surf ski race or 20 kilometres surf ski race there needs to be a much higher intensity effort.
In a race you may need to work up to 85% of your Max aerobic capacity, but if your body is used to working at 65%, then you can't effectively go work at 85% in that event. Lactic acid is going to build up quicker and you are going to fatigue and slow down.
It is important that you train your anaerobic threshold for the 10-kilometre event and if you were doing a 20-kilometre event you would need to train at aerobic threshold and aerobic capacity. Depending on what event distance you are training for, determines the heart rate zones you need to train at.