I did an 80 miler up to camp at a state forest on a 52x39 with a standard issue 7 speed in the back. 11- or 12-28, I think.
I'm tall and push big gears from atop my 66cm seat tube, even carrying heavy messenger bags filled with a 12 pack of longnecks or whatever. The last bike tour I took was around Ireland on a cheap gaspipe flea market bike when I was a rather out of shape 20 year old slacker.
Last year I turned in a good time at a local century, so I thought I'd have no problem just carrying lunch, water, and tools 75 miles or so up to a State Forest in Massachusetts from New Haven.
Wrong! I definitely headed up the canal trail way too fast and felt it as soon as I turned west into the hills about 65 miles in. The last 10 or 12 miles knocked me out!
Deep in the boonies, I was out of the saddle, pedaling out what felt like the last calorie out of my system... a friendly local called out "hammer!" from his porch, I made it a few more revolutions and pretty much walked up every hill I came to for the next hour. By that point, I think I was even going downhills in my 39x28!
It took 6 hours of riding time and about 7 and a half hours altogether. I felt like I bit off more than I could chew, but in reading some threads on here, many seem to feel that there isn't really any shame in walking hills now and then. It sure doesn't feel that way when you see the looks of pity from the drivers whizzing by.
All I had was a google maps cue sheet, and took a a wrong turn at the very end of the ride and descended an entire valley before asking whether it led to the state forest. Turns out both roads had the same name. At least I had the consolation of meeting two attractive women walking their dog at the bottom of the hill who then walked my bonking self back to their house and gave me a beer, a soda, and a bottle of water.
That made those last miles much better! Even the long hilly dirt/gravel road on my 23mm slicks.
I did manage to recover a bit and join our 4 and 6 year old boys riding their bikes around the campground that evening, though!
The route: http://maps.google.com/maps?saddr=We...h&z=9&lci=bike