planning a trip, got some questions :)

well at the minute im in a full leg cast, but im aiming to do the coast to coast bike ride on a unicycle next easter as a goal to get my fitness up for it and stuff :), its 140 miles long. now my problem is that i think if i was to get a 36er i would only use it on this trip, i really want to get into muni and was wondering whether anybody could tell me whether it would be out of my reach to do 140 miles over 4-5 days on a 26er? and what the average speed of a 26 inch uni is?