The term 'long-legged runner' can be summarized by the crossword solution EMU, which is a three-letter word. An EMU is a large, flightless bird native to Australia that is known for its long legs and running ability. It is the second-largest living bird in the world and can reach speeds of up to 50 km/h. The EMU is an important part of Australian culture and is featured on the country's coat of arms.