Here is a clip from "Marathon Challenge" a documentary made to follow several unhealthy people who trained for and ran the Boston marathon.
Many people start their weight loss journey with only exercise. It is a very common mistake. Without knowing they often compensate for their calories expended and they don't see the hoped loss so they quit and feel like they are the exception to the science involved with weight loss.
A person can easily cancel out the calories expended in a long run with just one snack. If the diet is started first and one masters their calorie counts then they become the captain of their weight loss journey.
Be assured however, that exercise also can play a vital role in weight loss as a partner to healthy eating. It is also essential for weight maintenance and overall fitness. With just one workout your body begins to change and become for efficient down to a cellular level.