The New York Times on diet vs. exercise for weight loss:
And in a just world, frequent physical activity should make us slim. But repeated studies have shown that many people who begin an exercise program lose little or no weight. Some gain. [...]
The implication, the scientists concluded, is that “active, ‘traditional’ lifestyles may not protect against obesity if diets change to promote increased caloric consumption.” That is, even active people will pack on pounds if they eat like most of us in the West.
You can’t out-train a bad diet.