As others have said: in order to train for altitude, you need to be at altitude.
12000 feet is not trivial. At 9000 feet, for instance, I myself cannot breathe. I don't feel right again until I'm below 6000 feet.
There are such things as 'altitude tents' you erect around where you sleep, that reduce the oxygen content to simulate altitude, but they're expensive and you're still not training *at* altitude. The only way to really do it is to go live at that altitude for a few weeks, spending the first week or so just existing and getting used to it, not even really training. But that's not cheap to do either for most people.
If it's just a one-and-done ride for you, you might just have to suffer through it. The 9000 feet elevation I spoke of earlier was a one-and-done for me, and I survived it.