Participating in a marathon is an activity that requires a great deal of physical and mental preparation. It involves running a long-distance race, usually 26.2 miles, and can take several hours to complete. The term for this activity is 'RUN, ' which is a three-letter word that can be used to solve the crossword puzzle. Participating in a marathon is a challenging but rewarding experience, and many people find it to be a great way to stay fit and healthy.