My brother John and I rode to raise money for a charity about 23 years ago. We only had 20" Schwinns at the time, so the 25 miles we rode was pretty painful. I recall that it took us 6 hours (with many necessary breaks, esp at the end) and that we rode together the whole time. Happily the ride was around the Central Park bike loop, which we lived a half mile from, so when it was done, we just rode home. Actually, that meant we rode 26 miles. The bath water stung me like I was getting into a tubful of Tabasco.
A year or two later, John rode with a 24" uni, and I recall that he managed to ride 50 miles, which still seems incredible (but keep in mind: He has since ridden in the recent AUT and two other long-distance rides, and that he has ridden 89 miles in 12 hours on a 29"). I’ll have to ask him for more details on that ride, but I know it was on a Schwinn 24".