It's possible to do a long climbing ride with no experience doing long climbs or lots of climbing in a day. But it's going to suck and you are greatly increasing your chances of failure.
Doing some big climbing days in preparation will tell you if you need lower gearing and will get you used to what it's like to have to pedal without coasting for a long time.
I've done more big climbing rides and races than I can count. I prepare by doing climbing. I'm fortunate in that there's some good 1500-2500' climbs near by but I'll drive an hour each way to get to the local 3200-3600' climbs, and then do repeats. I suggest doing hill repeats on your local hills and getting out to the mountains a few times before the event. Doing so will make the climbs seem much smaller.
Speaking of gearing, its better to have low gears and not need them than to need low gears and not have them. I'm a reasonably good climber but I run a 50/34 compact, and for some races I'll have a 30t cog on the back.