Cardio is only part of the equation. It's good that you are biking, but it's also necessary that you build muscle by doing strength training exercises three times a week. Also, it's necessary that you watch what you eat. Processed foods like foods made with white sugar and white flour, as well as fast foods (like a hamburger and fries at McDonald's or pizza) don't allow you to flatten your belly. You may want to look into a 90-day program you can do at home that is called RevAbs, which is put out by BeachBody. Great workout to tone up our abs. It also comes with a great nutrition program, so it's an overall fitness program.