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. Since he's running he's got that covered. But don't overlook the bike- at least if there's big nasty hills around- because with minimal impact to the knees/hips/back a guy can push his HR to its literal maximum (which is a WEIRD feeling) and then keep it there. I guess it could also be done via sprints on the flats, but, bleah to THAT. Hills are way more fun.
If your bike isn't set up for steep climbing, Ted, a (usually) simple swap of the cassette can address that. The biggest sprocket on my cassette (Ultegra 6800 11-sp) is a 32-tooth.... I can climb virtually anything...
