VO₂max — how much oxygen you can use at max effort — is one of the strongest predictors of how long you live. You can train it.
A brisk 20–30 min walk or easy jog where you can still talk but would rather not. That is Zone 2.
Higher cardiorespiratory fitness tracks with markedly lower mortality across every age and both sexes. Most gains come from consistent easy cardio plus a little hard interval work.
Low fitness carries mortality risk comparable to or greater than smoking or diabetes in large cohort studies; higher VO₂max associates with substantially lower all-cause mortality, with benefit continuing into the highest fitness groups.
Where it's thin: The mortality data are observational (fit people differ in many ways). That fitness itself is trainable and beneficial, however, is backed by strong interventional physiology.
Before hard intervals or maximal testing, get medical advice if you have known heart, vascular or lung disease, uncontrolled blood pressure, diabetes with complications, a recent major illness, unexplained chest discomfort, breathlessness, dizziness/fainting, or you are currently inactive and plan to jump straight to vigorous work.
Easy-to-moderate activity is safe for most symptom-free adults. Warm up, build duration first and intensity later, recover between intervals, and do not use an age-predicted heart-rate target as permission to push through symptoms.
Stop and seek medical assessment for chest pain or pressure, unusual or severe shortness of breath, faintness, a new irregular heartbeat, confusion or symptoms that are disproportionate to the effort. Severe or persistent symptoms are an emergency.