It can be very strenuous exercise, My experience is from a small club with a lot of small children so I guess they dial it down a little, but I still feel very tired and worn out by the end of the session. But not so much that i feel i'm going to throw up.
When you get into proper 'fights' it get so tiring though, I've definitely seen an increase in my fitness from doing this.
What i guess im trying to say is that you don't need to be very fit to take it up, you get as much out of it as you put in