It takes about 3 weeks, 3 times a week Walking for unfit clients to readlize a considerable change in their heart beat. A heart muscle that is hardly used for aerobic exercise due to a sedentary lifestyle looses capacity, just like a leg in a cast looses muscle.
It is amazing to see the change happen within only 3 weeks. Where as an unfit heart beats unstable, rises quickly and needs MANY beats to supply the body with blood during exercise, a fit heart muscle needs less beats to do the same job. If you use a heart beat monitor you can almost see your heart muscle grow. Plus: you experience a clear change in wellbeing breathing fresh air and relaxing.