I regularly ride with my bro in law on my old Raleigh Twenty(I upgraded the drivetrain with a SRAM dual drive 24 speed). He rides a litespeed tuscany with ultegra drivetrain. The thing is crazy fast. I stay up with him no problem. Would I race him on it? I don't think so, but I can easily stay up with him. You can ride any distance on a 20" folder that you would on a full size bike. If you're planning to do alot of 50+ mile rides, I would suggest some sort of suspension, though. Even if it is just a pair of Big Apple tires.
Juan