As above, fitness = cardio
I always found running the best when I did lots of martial arts, I just used to run places instead of taking the car, make sure you include lots of sprints though, as hard bursts of energy are needed in the ring, so an hours light running doesnt really simulate it very well but an hours jogging with random bursts of epic sprinting then dropping back to a jog does.

The other thing you might be getting wrong is bouncing around too much in the ring, its good to be light on your feet but IME people just starting out bounce about far too much just for the sake of it