I'm 34 and stay thin (BMI 21) by eating as much non-carb food as I want and doing strength training every couple days. My theory is that starving yourself and running long distances is counterproductive, because it teaches your body to be economical with energy, which is the opposite of what you want. And eating mostly fat is good for you, because it keeps hunger in check and teaches your body to use fat for fuel, which is exactly what you want. It's broscience, but it works for me.