I've been hunting deer up around 9k lately... I can say with some surety that being in really good aerobic shape greatly decreases the adjustment period/consequences of going low to high like the OP will be doing.
IMO, weight training done right is just as important. One of the things an elk hunter needs to be able to do is roll out of bed, every day, even after a tough day. Training your body to recover quickly from a tough workout that would leave most people sore is a big benefit. Cardio doesn't get me that.
