You can get fit doing the sport! General health is important for safety, you will get a medical form to fill in, it is a self-declaration form. If you have any significant medical problems (heart/lungs, diabetes, asthma for example) you would need to see a Doctor first to see if the sport was for you. Many people have mild symptoms that do not preclude them from the sport with a Doctors permission, however a serious condition would likely stop you. You will benefit from a basic level of fitness, but being slightly overweight or slightly unfit won’t stop you from doing really well in the course. It is something you can work on.