I ride a 42x16 FG, which is about as much as I can manage in the hills around here. Anything more than a hundred metres long with >10% gradient and I'm walking. I haven't yet done a century on that bike, but I've been over 70 miles a few times. Not sure I'd choose to tour FG/SS, but if I did I'd gear down to 42x18 or maybe even 42x20 to cope with luggage and more severe inclines. I agree with the OP about the fun element, though, there's a lot to be said for riding fixed.