When I trained for my Nar Phu Trek in Nepal 10 years ago, I did a lot of stair training up/down in a tall tower. It helped to some degree, but pretty hard to train properly for Nepal, all things considered. I do remember upon my return to sea level for a few weeks, I barely got winded or tired doing anything physical for long periods. It was a great feeling while it lasted. I've never been to Kilimanjaro, but heard good things about the trek and scenery.