You haven't really given us much information to work with, but based on assumption, it could be that you're compensating for the exercise done by involuntarily increasing your daily mean caloric intake. Running alone doesn't burn as many calories as you'd think, at 10km/h you burn roughly 500 calories per hour whilst running on completely flat ground, but this varies in accordance to your weight and other variables. This means that at most, you're burning about 250 calories each time you go running, but if you include walking and don't run for the full 30 minutes the number of calories you burn drops drastically. Despite this, your body still registers it as extra exercise thus making you think you need more food than you really do, after a run like this a cereal bar is often enough to balance out what you've burned. Also, remember that the average human burns 1500 calories a day passively, so you can assume that you are burning around 2000 calories a day if you run every day for half an hour, considering you probably also walk a few kilometers every day. Taking into account all of this, my advice would be to start counting your actual daily caloric intake, and once you have a stable figure of how many calories you eat on an average, start acting accordingly.One of the main things you must do to lose weight is to go in a caloric deficit. This doesn't mean you have to be taking in 1000 calories less than what you burn, but you definitely shouldn't be taking in more than what you burn. If you aim for a 100-250 daily caloric deficit it should be quite sustainable. To help you with this, start eating more low calorie per volume foods such as fruits and vegetables which will fill you up whilst maintaining your caloric intake low. If you eat animal products, stay away from cheese at all costs, and try to limit the meat you eat to low-fat meat such as chicken or salmon, as these are high in protein and have relatively fewer calories per volume than beef or pork. Coming back to exercise, try to increase the time you are actually running in that half hour. You don't need to be going very fast, a light jog is enough to keep burning calories, but if you stop for a rest or you start walking you are likely to end up burning fewer calories than you'd expect, especially because the longer you run without slowing down or stopping, the more calories you will burn per minute, so if you can't run for the whole half hour yet, instead of doing a mix of running and walking for that time, try to run for as much time as possible and consider the moment when you stop running the end of the work out. Make sure you stretch well before and after running and warm up a little bit to mitigate the chances of getting cramps or having to stop for other reasons. Finally, I'd consider trying other activities that burn more calories per hour than jogging such as swimming, and try to incorporate exercise into your everyday life as much as possible; start using stairs instead of elevators, walk to places near enough, and if you have a bicycle cycle if they're a bit further. This will help you burn more calories more calories per day on average, and it will increase your endurance and stamina for when you go running or do other physical activity.Sorry for the long answer, I tried covering as many points as I could with the information you have provided us. I hope this helps and good luck with your weight loss journey!