If you can sustain an HR of 165, then it sounds as though the issue isn't that you are going too hard, but that your Max HR is higher than you think. The (220 - Age) formula is an estimate, that doesn't take into account fitness or genetics (e.g. LA's would be the same as mine! ). Instead, you could use the 2x20 Anaerobic threshold test to get a better estimate, or find a center and pay some money to find out your VO2 Max.