You don’t have to be that fit to start kiteboarding. Compared to other water/wind-sports, kiteboarding is relatively easy to learn! The instructor will teach you how to control the kite, which is more about the technique rather than the fitness. However, the fitter you are, the easier it gets. It is recommended to stretch before each session.
For the advanced lessons, you have to be a Level 3 independent rider.
For the wave riding lessons, you have to be a Level 4 IKO kiteboarder and you must be able to jump, power jibe, jump transition, recover another rider, and understand weather and tides.