It's kinda like jumping in a hot tub. If you know nothing about a hobby and just sign up for classes it's going to feel like it burns. It helps to dip your toes in. Take a boot camp at a gym thats for beginners maybe read about the martial art you are interested in. So then atleast you have a rudementry idea of what's going to happen in class. We are all just robots with life times of conditioning. Even though we are past that age where everything comes easy you can learn. Humans are amazing, we are all capable of more then we think.