Ive been riding the stationary bike about 30-45 minutes per day for cardio and fat burning exercise. I set it to adjust the resistance to keep my heart rate at 150 BPM (i'm 22), usually i end up at about a 20mph pace. While my heart rate stays at 150, i dont have to increase my breathing anymore than I am just sitting at the computer right now. Does this mean Im not really going to be doing any worthwhile exercise or does it not matter?