A friend of mine did the southern route across the US from San Diego to Florida back around 1980 using a Mongoose Moosegoose single speed. It had 20" wheels but a larger frame so it wasn't meant for young kids. He did a LOT of training for the ride, many, many days over 100 miles. I rode my 26" Voris Dixon SS with him on a few centuries and he was well prepared. He finished the ride easily, I forget now how long it took but it seemed pretty quick to me, like a month or something. My answer to you is that you can do a ride on just about anything but conditioning is the key.