Brad, I am not an expert but I have learned a lot from coaches at our portfolio company, Carmichael Training Systems (CTS). As you do so many marathons, your body adapts to the long distance training and you don't continue to lose weight. As CTS coached Dean Karnazas for his 50 marathons in 50 days and other marathoners as well, there is a need to do interval training at higher heart rates. The weight loss will continue as long as you change up the training so your body is continually adapting to new training.