How to improve cardio?

I used to be decently athletic, I played jr A hockey and we trained to improve are cardio nearly everyday so i know some exorcises but to be honest I am not wanting to go to those extremes lol. I run on my tread mail four times a week for 30 minutes before my workout however I have not noticed any improvement in my cardio and i have been sticking to this regiment for about 7 months. I am not trying to become lance armstrong but I would like to get back in shape, i am not trying to lose body fat. I have a tread mail, bike, and stationary bike whats the most effective way to increase cardio.