Your profile says you live in Springs. Assuming that's true, you have some of the best training grounds around! To train for elevation gain, there's nothing better than doing the activity that closely resembles the goal activity.
You have the Incline, which I wouldn't do with a weighted pack, but, rather, hammer up it as fast as you can, even if it is slow. I'd do that just once a week and, if not, every two weeks.. Personally, I wouldn't descend it, but take the Barr Trail back down and repeat it one or two more times, if you have time. And, since you're in the neighborhood, load your pack and do an out-and-back on the Barr Trail as far as you can go, when you have the time. I don't know what the trail conditions are like right now, but the higher you can get, the better (i.e. at least to the A-frame which is just under 12k). Even, if at first, you can only get to Barr Camp. You may also want to consider doing Section 16 counterclockwise, since you get the steep stuff over with first. I would physically push yourself on these hikes--not only on the climb, but the descent, too.
Flat Top is seriously flat on top for a little bit before you begin descending to the West. The climb up isn't that bad and then it's nice and flat, although rocky. Like someone else has already said, you're already at 6k to 7k already, and 12k isn't going to be that big of a burden...