I just got back from three hours of kick-boxing, and am pretty exhausted – "cream crackered" as they say over here. It's the first time I've been in over a week, since I had a cold for much of last week.
Apart from the obvious physical demands, kick-boxing is quite demanding mentally, at least to the beginner. It's a little bit like being a beginner at driving, in that there are many things you have to remember to do correctly all of the time, there is no time at which you can forget any of them.
You have to keep your guard up at all times, fists clenched at your temples. Your fists have to be clenched properly and tightly with the thumb tucked away over the fingers. You must keep shuffling: moving around, bouncing, bobbing, weaving at all times – even when listening to instructions. But you mustn't shuffle during a combo. During the combo, pivot for reverse punches, hook punches, uppercuts, side kicks and roundhouses, but stay straight for front kicks, jabs and back hands. Ki-ai (i.e. shout, for me it's sort of a "Tsuh!" sound) at the end of your combo. Keep shuffling. Don't punch or kick too hard, just "sting" the pad / sparring partner. Return hands to guard as quickly as possible after a punch.
It's a lot to keep in mind. The nice thing is that, like driving, more and more of it becomes automatic as you keep doing it. Still, I'm a bit worried that I'll have a brain-related failure during my grading on Sunday week.