So, take the following with a grain of salt, because I personally don't really believe it myself.
I'm currently in a physiology of exercise class for a bachelors degree in exercise science and my professor recently stated that you're only gaining a training effect for the first 2 hours of exercise. Anything after that is a wasted amount of time. Again, this is what he said and I don't personally believe it myself. He pointed out that Intensity, and Frequency was what mattered, and that the duration, and Mode did not matter as much. Interesting to say the least!