Cross country???? Jeez you've got my respect. That is & probably always will be just a dream of mine. Any man who actually has ridden a roadbike from coast to coast is one tough hombre IMO.
Cross country just means off the road, sometimes on paths, sometimes on open ground where there is no path. It does not mean riding across a country.
That said, there are plenty of people who have done either the coast to coast or the end to end (Lands End to John o'Groats) on unicycles and thousands who have done it on conventional road bikes. One bloke recently complete a round the world ride on a unicycle.
At 56 years old, and with many other hobbies to fill my time, my biking and unicycling tend to be limited to single day rides of a few hours. However, I divert off the road onto paths and open country whenever I can.