Getting in shape for snowboarding/backpacking

[Nefret]

So when I exercise my main regimen just consists of running a few miles and doing some sit ups. I've been doing this steadily for about a month and a half every other day but I'm starting to get some stamina and I feel like I should actually get an activity specific routine going.
I snowboard every other week and I'm planning a 3 week long backpacking trip in about 1-2 months that I'd like to be in better shape for. Anybody have suggestions on what I could do to get my routine more specialized for these sports? Being female I've never really gotten into weights and whatnot but I can always start sometime and my only cardiovascular is running.

[The English Gentleman]

Height and weight would help.

Of the top of my head by snowboarding friend said core body muscles were important for balance. Thus balancing on a balance board for twenty minutes a day would be good, along with plenty of situps.

[HLC]

Wear a loaded backpack and climb stairs.
Also do step ups on a chair or bench, with the pack on.
And work your repetitions up to a crazy high number, like in the hundreds.
A backpacking expedition will have you taking 2000 steps or more per mile, so doing 10-12 reps of something for preparation isn't going to do all that much for your needs.