This feeling is also common to other martial arts as well as other forms of exercise including (but not limited to) long distance running, lifting, a lot of military basic training, etc. This is not meant to belittle it; it seems that at least you're getting a workout.
I could barley hold my hands up after about 50 reps, i was gunna collapse. all of a sudden i just felt this "im not giving up!" feeling pass over me. I switched on. and hit better than i had in the 50 reps before. it was an amazing feeling.