i did a metric century (~61miles) on a single speed. not hilly by most parts of the world standards.
i think i didn't really notice not having more than one gear due to the cold rain.
and once i bonked around 45ish miles, i took a nice fish & chips lunch and a couple pints of stout. after that the cold rainy ride home was no more miserable than the rest of the ride - but the whole deal is obviously quite memorable!