I concur with Ellen's routine. Last year before my first Whitney summit, I started out with San Jacinto from the tram. That was a good altitude intro with plenty of mileage.
Then I did San Gorgonio via Vivian a couple weeks later.
Then San Bernadino 2 weeks after that. This should have been before Vivian, but whatever.
Whitney was 12 days later. In between I do short hikes on local coastal trails to keep in condition.
Before my second Whitney summit I did Vivian 1 week before and I did ok.
I think San Bernadino and Gorgonio are really good conditioners for both altitude and physical stamina.