What do I have to do to prepare?

The recommendation is to try and increase your fitness, increase the number of paddling sessions you currently do. Also to practice paddling the boat you wish to race in.

A good recommendation is to check your equipment, make sure it is all water tight and if in doubt speak to the race organiser. That way you can confirm where the start of the race are, best way to get there, (if you are unsure). Etc…