The very best way to prepare for climbing to high altitude is to climb to high altitude. Although this is difficult for most people, remember that the higher the altitude and the longer the hikes, the better. Whether it be to hike 2,000 feet to the top of the local hill or climbing larger mountains further afield; just being out hiking up hill is the best. This also allows you to get used to your backpack and boots.
Running and biking are also very good and at least take you outdoors so you can test your equipment. Stair masters and climbing machines at the gym will work if outdoor hiking isn't an option. Try not to go for short hard blasts of exercise but long sustained workouts instead. An adequate training regime is to maintain 80% of your max heart rate (220 minus your age) for an hour, three to four days a week. Remember high altitude mountain climbing requires acclimatization and a strong mind is as important as a strong body. If you are fairly fit and choose a climbing itinerary that allows plenty of time to acclimatize you have the formula for success