Everyone's chasing HIIT classes, personal records, and that post-workout dopamine hit — but the most robustly supported longevity exercise in the peer-reviewed literature looks embarrassingly undramatic. This week, we dig into Zone 2 aerobic training: what's actually happening at the cellular level, why it's nearly impossible to do correctly without a heart rate monitor, and why the fitness industry has a financial incentive to keep you gasping instead of cruising. No tracker required — though after last week's sleep score autopsy, you might be side-eyeing yours anyway.