Weight loss/gain seems to be a maths thing, by and large.
If you take in an extra 350 calories a day (say, about a Mars bar worth each day) above what you need, you will be putting on 3500 calories every 10 days (=1 pound of fat). That pattern, if multiplied throughout the year, would equate to 127,750 calories (i.e. 350x365 calories) over a year, which in turn would equate to 36.5 pounds of extra weight (i.e. 127,750/3500). So, just a small amount of extra calories per day above what you need (350 calories per day, in my example - easily done by any of us) would, at the end of a year, give you an astonishing 36 and a half pounds in extra (unwanted) weight.
The converse is also true (which is why running works in trying to reduce weight): A calorie reduction per day of 350 calories (easily achieved by anyone in a "half hour a day" run) will reduce a runner's weight by about 36.5 pounds over the year (in fact, a little less than this for the reason that it takes fewer calories to maintain a lower body weight, so, as weight decreases, the body's daily requirement of calories will also decrease).
Runners who want to lose weight but find that their weight has stabilised do need to look more carefully at the calorific content of what they are eating. The probability is that, although they are running off a certain number of calories by exercise, they may be increasing their eating correspondingly (maybe even using the fact that they are running as an excuse to eat more - the reward system we have!) which is cancelling out that potential weight reduction.
Although you may burn off, say, 400 calories on a run, unfortunately that isn't very much in food/fat terms in any one day(say, the equivalent of a chocolate bar or an extra portion of potatoes or 6 slices of bread) so the runner who wants to lose weight STILL has to be careful about what he/she is eating - which does mean a certain amount of counting of calories and being aware of what he/she is consuming. It is the gradual accumulation of a long series of days where the calories consumed are less than those burnt off which leads to the weight loss.
It seems to be a truth that we don't actually need very much food to keep us ticking over quite nicely - which is unfortunate if you like food and if you like big portions. The human stomach can hold about 30 fluid ounces (about one and a half pints) in terms of quantity of food, but we don't need to fill that container, and certainly don't need to do so at every meal - we are becoming a nation of obese people mainly because we have ample amounts of relatively inexpensive calorie rich food around us and we are not careful enough about making sure that we only consume such amounts of it that we actually need. So, we wake up one morning and ask ourselves how we ever got to be this big (probably because we have been eating too much and have failed to be sufficiently active to deal with the excess intake of calories) and we then, when we are ready, have to embark on the long reverse path of undoing the process by calorie reduction - by eating less and being more active.
Anyone who wants to lose weight has to prioritise it and practice good eating habits with restricted calories. If people run and still restrict their calories to a sensible, the weight is guaranteed to fall off, inevitably, in a very satisfying way, but they do still need to concentrate on restricting their calories. Exercise alone will not make a person thin if he/she has a natural tendency to overeat and is not prepared to watch the calorific content of what they are eating/drinking.
