I'm amazed that all these sub-200 pound runners are worried about being in shape! I'll drag myself up the mountains this summer clocking in somewhere between 225-240 depending how much weight I can cut over the next two months.
I'm with you on that, I train Muay Thai so I'm in pretty good (6'4", 230) shape, but I'm no slight little runner. Here's to being big and slow.
My advice is to hike bigger hills in Spokane with a noticably heavier pack than you plan on using and try to push the pace a little to build up the cardio to compensate for the lower red blood cell count.
Also concur with CO Native, avoid the meds as much as possible coming up from lower elevations. I do usually take something before hikes but I'm familiar enough with my body at altitude that even with the meds I can determine what is safe but slightly uncomfortable and what is a danger sign that should be listened too. I have seen people over medicate and then have issues because they were literally numb to the warning signs.