Yes, you can lose weight by simply adding exercise to your daily routine and not altering your diet as long as your your walking will put you total calories in deficit. So (as an example only), if you need 2000 calories per day just to maintain your weight and you are consuming 2500, then your daily walking will have to consume more than 500 calories in order to see any weight loss. How much walking you would need to do to lose those 500 calories per day depends on how much you weigh and how fast you walk. If you need to walk off more than 500 calories then you will obviously need to walk that much more. I can tell you though, that this kind of weight loss regime will be either tortuously s-l-o-w or you will become a marathon walker.