Exercise training in cardiac rehabilitation (rehab) is a key part of managing a patient with heart disease. It has been shown that cardiac patients who increase their aerobic (cardio) fitness by exercise training live longer, have better quality of life, and stay out of hospitals more than patients who do not improve their aerobic fitness. The more a patient improves their aerobic fitness the greater the benefit. But it has been shown that more than half of patients do not improve their aerobic fitness even after participating in cardiac rehab. This may be related to how hard patients are asked to train (their training intensity). The way intensity is chosen in current programs is commonly based on a one-size fits all method that may not consider that different patients have different abilities. There are more personalized methods to determine training intensity that exist, but these have never been used in cardiac rehab. One method divides intensity into three zones (zone 1 = moderate intensity; zone 2 = heavy intensity; zone 3 = very high intensity) that are based on when an individuals' biological responses to exercise change. The purpose of this study is to see if this approach gives better results in terms of changes in aerobic fitness and if training in the different zones makes a difference. Three groups of patients will be asked to train for 3 months in one of the three intensity zones. Aerobic fitness before and after exercise training will be compared to see which intensity zone results in the largest change.