A day might come when you realize that you are abusing your body by over-eating or not eating the right food. Even your body starts showing signs of deteriorating, and that will be high time you need to follow fitness regime and a new lifestyle. And believe me it's not that hard to change, you will love your body and thank yourself for embracing this change earlier than never.
To tell you my story, I am a yoga person more than doing exercises/walking/gym. As I grew older, i got very less time to do yoga or meditation. I made my yoga regime very short day by day - ended up just doing Suryanamaskar in the morning for about 15 minutes. But seriously I did not do anything else to care for my fitness, and it started reflecting in other ways - you can call it putting on weight, medical check up results rising up my usual notch and what not! It was high time I had to start my fitness regime and care for my body. It's a long long way to go... more than half of my life to live.