I am punishing my cardio, trying to get develop wind at sea level I will need for 7 days at 11,000 feet in Colorado in a month. I have tried several things - here is what works best for me. Interval training seems to work the best for stressing your cardiovascular systema and getting your body used to working while sucking wind. One way to do this is outside on the track - sprint at top speed for 2 mins, then walk or jog for 2 mins - repeat up to 8 intervals. I have found as I have made some good progress getting my wind up, I'm having a hard time pushing myself, so I have moved over to the treadmill. Current regimen - stretch - get on treadmill 7 degree incline at 3.5 mph. After you feel warmed up and loose, do the 2 min interval, running up the incline, start at 6.5 mph, lower mph as you run out of air completely, but do not decrease incline. That running uphill is punishing, it definitely gets me to my maximum. Do 8 2 min intervals, even if by the end you are barely jogging going uphill. If you do the latter one for a couple weeks you will see a noticeable increase in your wind.
I'm also climbing the parking garage next to the hospital near my house, its only 9 stories, but I take double stairs with a 40 lb pack on. I do double stairs to my goal, which I try to increase every time, then when I can't do more of those, I do one rep single stairs, then one last rep with the pack off. You really fly right up when you drop 40 lbs!