You eat to provide energy to your body and nutrients to keep your system working at the optimum level. The calories that you put into your body should match the calories that you burn through your daily activities. That's one of the reasons you need to exercise--to burn calories. But there are even more compelling reasons.
Exercise is linked to your cognitive functions helping you to be more alert, to your emotional well being leading you to become more positive and less depressed, and to a healthy body that works smoothly and efficiently . Exercise, too, lowers blood pressure and eases the effects of diseases on your system a thing of the past.
All you need to do is choose the exercise that suits you best. That could be a walk, gradually working up to the level till you're walking twenty to thirty minutes a day. Or a swim. Or sit ups in your bedroom. Or any activity that allows you to be committed to doing it everyday.