How does light regular exercise keeps one fit?

Read the passage and answer the question that follows.

Most people don’t mind working out a bit, but are discouraged by the thought of exercising hard. The idea of sweating buckets is so daunting that they avoid exercising. 

Fortunately, exercise is not a matter of all or nothing. There is some reward at every level of effort, provided it is regular. Spend at least 30 minutes a day in action like brisk walking to errands (not between rooms!) climbing stairs, carrying heavy shopping or children, doing outdoor gardening. Your rewards will not be immediate, but you can expect a reduced risk of problems like brittle bones, heart disease and poor blood circulation. You work up a decent appetite and sleep well, but improvement in body-shape will be minimal. 

Light regular exercise, three times a week, makes your heart run faster for at least 20 minutes. Such aerobic activity can include anything from brisk walking, jogging, cycling, swimming, any light sport or even disco dancing. The reward is immunity to colds and flu, less stress, slightly healthier levels of cholesterol and
blood pressure. The figure trims down a little but only gradually.

Not being afraid of working up a sweat, put in some aerobic activity, longer than the minimum 20 minutes. Anything up to an hour, 3 to 5 times a week. In addition, do floor exercises, like push-ups or yoga and some light-weight work to exercise different parts of the body. Your rewards are substantial you have high resistance to chronic diseases, infections and your heart performs strongly. You look fit and clothes hang well on you. You feel youthful, do not get breathless easily, and have good muscle strength. You have the stamina to participate in adventure sports like trekking and mountaineering. You perform better in regular sports. At work, your mind is clear and optimistic.