"You can’t out-exercise a bad diet." – Dr. Mark Hyman, a physician and advocate of functional medicine.
It's true... I've tried that and saw zero results. I'm a cyclist. Some of my bike rides take up to 5 hours. I have a heart rate monitor that estimates my calorie usage. I often see 2000 - 3000 kcal burned on a single ride. And even with that intense training, I couldn't lose weight. Now I'm on a low-carb, high-fat, and high-protein diet. A lot of meat and low-carb veggies. My belly is gone. And I didn't even have to do intense training. DIET is the most important thing. After that comes physical activity.