If your aim is to lose weight, you should not be running too quickly. In other words the intensity of your runs should remain at 60-70% of your maximum heart rate. You can work out your maximum heart rate by subtracting your age from 220, or by using devices designed to provide you with heart rate information. Alternatively, you could train according to your levels of fatigue. This means analyzing how much of a conversation you can hold while running. To burn fat effectively, you should be running for 30 minutes or more, 3-5 times per week.
However, these groups of runners should also be thinking about their diet and weight training to strengthen muscles effectively in a way that also burns fat at a fast rate.