Well done for trying! 3400 ft is about the amount of climbing I generally do over 60 miles, so packing it into <2/3 of that distance is a lot!

I get your desire to do it solo, after being around people all week. BUT in a supported Century ride, you can ride alone - that's how I did the one Century I have ridden. The support is great because there will be food and drink, other people struggling just like you, and a well-planned route. So consider doing that for your first one. The one I did, I amazed myself by not losing any weight over the ride. At the time, I thought that meant I oveconsumed at the rest stops, but I think that was actually key to being able to complete it without collapsing from dehydration or low blood sugar. I was stiff and sore the next day but not cripplingly so, and I think a lot of that was keeping hydrated and fed.

Regarding hills, there's not much other than riding hills to train you for riding hills. You can get gears low enough to climb up a wall, but it's still hard to put in sustained efforts for half an hour or more without building up to it.