Most guidelines recommend at least 150 minutes a week of moderate aerobic activity (such as brisk walking, cycling or swimming) or 75 minutes of vigorous activity, plus muscle-strengthening exercises on two or more days. This lowers blood pressure, improves cholesterol, helps weight control and reduces the risk of heart attack and stroke.
You can break it into short sessions, and even small amounts of activity bring benefits, especially if you currently do very little. Reducing long periods of sitting also helps.
If you have heart disease, chest pain, are very unfit, or are starting after a long gap, talk to a doctor before beginning vigorous exercise. Stop and seek help if you get chest pain, severe breathlessness, dizziness or palpitations during activity. A graded approach, building up slowly, is safest.