Begin by walking round the garden (yard)... progress to round the block, then as your stamina is restored, increase the distance gradually until you can walk for several miles with confidence.
Above all, don't push yourself beyond your limit. It's tempting to say "I'll do a mile!" -..... but if you're knackered after 400 yards, STOP. The extra distance won't go away, and you can do it later.
Always take some sugar/carbohydrate
such as a Mars bar. Just in case.