Hi Steveyo,
I have been building up my endurance and my longest rides with no breaks are probably over 1/4 mile. I have not tried to measure them accurately, but I have a loop around the street that is 0.13 miles (about 210 meters) that used to be impossible and is now fairly easy. I will say that ultimate wheel riding builds up thigh muscles!
I do not know that it will ever get easy. I note that the UDC 28" ultimate wheel has a “crank length” of 160 mm, and that it would probably be easier to cruise if it were shorter, perhaps 120-140 mm. It would be harder to ride but the speed and perhaps the muscular effort required might be less.
Scott