When I hunted CO, the elevation varied from around 7K to up past 13K, and for a flatlander from Texas, it was tough going. At the time I did a LOT of work on cooling towers where I climbed stairs a dozen or so times a day to get tools and what not. I added in my boots, and made sure that each trip I carried up something that weighed at least 15# so if nothing else I at least had my rifles weight covered.
We headed up a week ahead of the opening to scout and find an area we liked, after we got up there, the altitude sickness hit me the first morning and I can tell you it was far worse than any hangover I have had. At least with the hangvers I still had something to ride the effects of, with the altitude it was straight pounding with no relief. It luckily only took heading own to around 9K feet for a few hours to get over it, and after the first day out I was good.
All of the other comments are good but two additional things that will make your trip up high WAY better are some saline nose spray and chap stick, might throw in some lanolin based hand lotion as well. Those things were given to me by a local when we got there along with a bottle of Advil, all of which came in handy while we were there. They advised to not take the Advil while up high for reasons mentioned above but to use if when we got back down if symptoms persisted, and to check in with the local Dr if things didn't clear up within 24 hours. Luckily everything was good, but the chap stick and nose spray came into use after the second day up. The dry air had my sinuses almost bleeding, and my lips in just about equal shape. Just a little bit works great, but I would have hated to have not had that friendly handout.
As for getting in shape, the one fellow told us the best way he knew for us to get ready for the following year, was to put all our junk in our packs, sling up our rifles and hump it up there on foot in a fast jog. LOL Unfortunately we never got to try out his advice.
Good luck with your trip and your new additions. Myself I quit after one, but could have just as easily added another. The second wife decided that one was enough and I respected that.
Oh yea forgot to add this, be SURE you have a roll of duct tape, and a can of spray antiperspirant with you, and also that you wear two pairs of socks, one of those thin nylon dress types and an outer wool type that will wick. This will help keep your feet dry as well as add in cushion. If you find your feet sweating to the point the outer socks are actually wet, try spraying them with the antiperspirant before adding the socks. Take about 3' or so of duct tape and find you a pill bottle that is wide enough to roll it around. Roll it around the bottle and stick it in your day pack. If needed, you can pull off 6" strips to put in behind the heel or anywhere else on your outer socks that you might be getting a blister from a rub. If you find it happens once, then you can put it on in camp before heading out, but it will save a days hike if needed while out there. The tape also comes in handy if you need to mark a trial as well, just pull off and stick to the tree bark as needed, just be sure to pull it off on your way out.