I basically just get out of breath. I am usually around 300-350 watts avg. over the entire ride and usually avg. about 18-22 mph. This puts me at about 165-175 avg bpm heart rate at the end of the ride. Good news is if I do intervals, I can recover very fast. I just want to be able to "push it to the next level" and go faster and harder, but feel that I am hitting a wall called my lungs.