Having just gotten back from about 100 miles out at Philmont (Scout ranch in NM), I would claim that my fitness regimen works reasonably well...
3 days/week: 2-mile loop, walking in 26-28 minutes.
1 day/week: 30 minutes on my Nordic Trak
1 day/week: 30 minutes swimming laps in the rec center pool (about 0.8 miles)
The couple of weeks before Philmont, I substituted/added some cycling to that...covered maybe 100 miles over 10 days, one fairly long one (25+ miles), a couple of 16s and the rest 5-10 at a time.
I should probably add some strength conditioning to that, but the primarily-cardio mix seems to work pretty well for normal backpacking and mountaineering. (I'm no spring chicken...well on the far side of 50...)