Training for a marathon is an intense physical and mental challenge that requires dedication and commitment. It involves gradually increasing the distance and intensity of running over a period of time, as well as incorporating strength training and stretching into one's routine. The goal is to build up the body's endurance and strength so that it can handle the demands of running a marathon. The RUN solution in the crossword is an important part of this process, as it is the primary activity that will help one prepare for the marathon.