Ponzy: What Snacking Bear said.
12 hours on Half Dome is on the slow side, but you said you were taking it easy. You will likely want to pick up the pace on Whitney. I'd rate Half Dome at about 60% of a Whitney hike.
I might do Half Dome as a warm up hike for Whitney, not for the acclimation, but to get the legs warmed up to the long distance. If you can increase your loops so that you can do 16-20 miles in one day, then you will do pretty well on Whitney. The one thing you might consider in addition, is spending two nights at Horseshoe Meadows before Whitney to get yourself better acclimated to the altitude. Please understand: You are starting the Whitney hike at the elevation of the top of Half Dome. Altitude is a big factor on Whitney, and not at all on HD.
Beware: You have 6 more days to pay for your permits, or they will be cancelled on Apr 30. I would pay for 1 and let the others go. You will find plenty of friendly hikers on the trail.
