I think all of these "rules" and figures that are being quoted to Catrin might be enough to scare anyone off, even if they were just thinking about doing longer distances.
Frankly, I find the key to surviving long distances with lots of hills is to climb slowly, on purpose. I don't care if I am going 4.5 mph, because I know I will get up the hills, and I also will finish my ride not feeling like I am ready to die.
I know brevets have a time limit, but most centuries don't. But, often, I get to the end of a long, hilly ride before the people who were going up the hill faster than me, because somewhere along the way, they ran out of gas.
Catrin, I think you can do it. Increase your mileage slowly, as you plan to, experiment with your nutrition/hydration, and ride hills. And don't underestimate how much of this is mental. I never ride really long climbs at home, but I was was able to do many 5-10 mile climbs, with serious gradients on my tour, because 1) I ride lots of short steep climbs at home and 2) I knew I would have to do this, I made up my mind I would and I wouldn't even consider getting in the support van.