Dr. Steinberg said that marathoners have found tapering off their runs — not doing major long runs a few weeks prior to the race — is a good idea. As one marathon coach and co-director of the Portland Marathon Clinic explained to Runner's World, "In those last few weeks, it's the rest more than the work that makes you strong. And you don't lose fitness in three weeks of tapering. In fact, studies show that your aerobic capacity, the best gauge of fitness, doesn't change at all."
But, Dr. Steinberg did recommend, especially for first-timers, doing a half marathon a month or six weeks prior to the marathon, "just to kind of get an idea of your nutritional requirements and see how you fare so you can set realistic goals."