Was that your first class?
Was that the only class you went to?
Here there are four levels of Krav Maga. All the levels may be mixed in a level one class, as level one includes a LOT of conditioning to get you street fight ready. Lots of people myself included are coming in pretty out of shape, and as was said, that can lead to injuries.
So yes, to get good at Krav, ANY Krav class is going to condition the bejesus out of you!!
The bat and knife and gun defense training will come later...
And yes, the small female instructor holding the bag is there for a reason. How many punches do you need to throw before you have muscle memory? Correction- how many punches with proper form do you need to throw before it becomes automatic?
Those punches against the bag she is holding is conditioning your knuckles.
I'm not sure if the class you did was legit Krav or not, but it sounds like you went to one class, had to do some intense conditioning (which any place should have you doing), and some basic stuff and you were sad because they didn't make you fight 30 ninjas your first night...I would go for a couple months, if you aren't rolling by then maybe you're right.