Plan cardio duration and intensity.
Planning cardio needs two answers: how long, and how hard. Duration comes from the guideline you are working to — commonly 150 minutes of moderate activity a week — divided across your sessions, with one adjustment that surprises people: vigorous activity counts double, so 75 minutes of hard running satisfies the same guideline as 150 of brisk walking. Intensity is set by heart rate, and the Karvonen method is the one to use, because it works from heart rate reserve rather than from maximum alone. That matters because a fit person with a resting rate of 50 and an unfit person at 75 have very different zones despite identical maximums. Maximum itself is better estimated by 208 minus 0.7 times age than by the familiar 220 minus age, which systematically underestimates older adults.
Karvonen target zone and session length
Max HR = 208 - 0.7 x age; reserve = max HR - resting HR; target = resting HR + reserve x zone fraction (0.5-0.7 moderate, 0.7-0.85 vigorous); session minutes = (weekly target / intensity credit) / sessions, where vigorous credits double
Heart rate formulas are population estimates with an individual variation of around ten to twelve beats per minute, and medications such as beta blockers change them substantially. If you have a cardiac condition, are pregnant, or have been inactive for a long period, get medical clearance before beginning vigorous exercise.
Because public health guidelines are written in moderate-intensity equivalents. The evidence shows one minute of vigorous activity delivers roughly the benefit of two moderate minutes, so 75 vigorous minutes meets the 150-minute recommendation.
Because percentage-of-maximum ignores fitness. Two people with the same maximum but resting rates of 50 and 75 need different target rates for the same relative effort, and only the reserve method accounts for that.
Use the talk test. Moderate intensity means you can hold a conversation but not sing; vigorous means you can manage only a few words at a time. It correlates well with the calculated zones.