The ERASE Trial will be a phase II randomized controlled trial. A total of 66 men with low- or intermediate-grade clinically localized prostate cancer undergoing active surveillance will be randomized to either a 12-week of supervised high-intensity aerobic interval training or usual care. The primary outcome will be peak oxygen consumption (VO2peak) as a measure of physical fitness. The secondary outcomes include tumour-related biomarkers, fear of cancer progression, quality of life, and psychological distress. Exploratory outcomes will include indicators of cancer progression including prostate-specific antigen. Based on this sample size, our study has 80% power with a two-tailed alpha <0.05 to detect a clinically meaningful effect of 3.5 ml/kg/min on our primary outcome of VO2max. This power will also be sufficient for detecting differences in our secondary biomarkers and patient-reported outcomes if the effects are at least moderate (i.e., a standardized effect size of d=0.60).