Participating in an ultramarathon is an extreme physical challenge that requires a great deal of dedication and training. It involves running a distance greater than the traditional marathon length of 26.2 miles. Ultramarathons can range from 50 kilometers to over 100 miles, and can take place on roads, trails, or even across deserts. The term 'RUN' is an appropriate three-letter solution for this activity, as it is the primary action involved in completing an ultramarathon.