For many years I did a similar kind of thing - I'd be training at home on a trainer in CT and then head out to SoCal and try and get to the top of Palomar Mountain on a long ride (about 100 miles round trip, climb is in the 15 mile range if you count the "pre-climb" as well as the actual climb). I happened to do a similar trip, again coming from a flat terrain and trainer regimen in CT and then going to Edwards CO and doing a big ride out there. As a distinct non-climber and non-enthusiastic rider on the hills in CT I almost never do hilly rides at home.

You'll be working at threshold on the climbs so you want to work on that, in terms of fitness.

For your bike make sure your lower gears are in working order. Since I don't spend a lot of time on hills my lower gears (39x25, 39x23) don't get used a lot. Each year in SoCal I had to fine tune my rear derailleur because I'm in foreign-to-me gears on the long SoCal hills.

It'll be beneficial if you don't coast a lot - climbing is consistent work. Trainer rides really reinforce constant pedaling; outdoor rides encourage a bit of coasting, especially if you have even minor hills. I prefer trainer rides for many reasons and that's one of them. If you can do 1-2 hours on the trainer with maybe 5-7 coast breaks (if you have a downloadable cyclocomputer Garmin etc you can see where your cadence goes to zero) then you're going to be okay pedaling for 12 miles straight.

Finally the biggest thing you can do is to lose weight. I went and did the Palomar thing for 7 years, going to SoCal for 1-3 weeks every Jan/Feb. I was super consistent going up Palomar except the year I showed up 30+ lbs lighter. Then I was 15 minutes faster, give or take. I'm not a climber so it's not like I'm fast, but I put down less power and went faster when I was lighter.

To put my weight in perspective when I first went out to SoCal in 2004 I was coming down from weighing 215 lbs over the winter; I was probably 200-205. Most years I went out there weighing 185-195 lbs. My light year I was 155. I'm 5'7".

When I went to Edwards it was that "light weight" year, visiting a former teammate and good friend that lives out there. Although I'm a sea level resident I had no problems at the base altitude of 7000 feet. We went over some climb, maybe 9000 feet?, and in the end of the day long ride I had played it so conservative (thinking about all the stories of how altitude is so hard) that I did a monster effort at the end of the ride, going super hard up a steep 1/2 mile hill. I learned the hard way that it's not that it's hard to go fast, it's hard to recover from such efforts. My effort in Edwards would have taken me maybe 60 seconds to get over in CT before I could pedal normally. In Edwards it was something like 5 minutes and I thought I was going to pass out from lack of oxygen, it felt like I was breathing into a garbage bag.

It'll be a change, that's for sure. I think you'll have fun just because it's different. Good luck!