Anything you read about planning to hike around Mount Blanc has one bit of advice with it: "You can not do too much cardio prep." I've hiked a bit in the Evergreen, Colorado area so I know what they are talking about. Exercise at an elevation, 7,500 ft on the trails around Evergreen, does put pressure on one's system. It's just plain harder and most of us go slower and rest more.
Since I pretty much live at sea level - I'm rather sure those rivers I can see as I walk to work in Manhattan each day flow directly out to the ocean- I've been living on the stairmaster. Nightly workouts have the stairmaster up at the maximum incline and me going at it for about 3 miles a night during the week and about 8 on weekends. I hate it! I really hate it! I just really hoping it is going to pay off in a couple of months.
Anyone who really knows me realizes that I have long since given up being first at any athletic endeavor. I have plenty of family members who have spent a lifetime beating me at all sorts of sports - and now the next generation is coming up and doing the same thing. I just really don't want to be last and am willing to struggle through nightly stairmaster workouts to prevent that.