The best way to train to climb taller mountains is to start off by climbing smaller ones. E.g If you wanna get on top of everest, you need to feel strong and fit at the top of the 100 or so other climbs leading up to Everest. I was given this climbing advice by Dave Hahn, 11 time Everest climber. (World record for non-sherpa)
I was recently climbing in Ladakh, Himalayas at an altitude of 4000m. It is tiring for first timers, but i'm an avid mountain biker and low altitude climber in the US, so it's pretty easy for me, and my build is what you would call 'Bearish' - broad and tall.
It really depends on your fitness level, your cardiovascular system, and your strength. You need to be able to carry yourself while spending part of your energy just breathing, especially in the Andes and the Himalayas, and it doesn't help if you are prone to altitude sickness/ Edema.
A pair of nice hiking boots which will save your feet in any normal hiking situation is the Lowa/Vasque goretex boots. The Lowa is lighter whereas the Vasque will protect you better, your feet will be well insulated.
HOWEVER, if you want to trek at lower altitude just get a pair of Northface hiking shoes, they're extremely breathable and don't cost more than a hundred singapore dollars for a decent one. (I paid US$180 for the Vasque goretex).
Trekking at lower altitude - the more you do the easier it will be. If you get tired somewhere like Bukit Timah, for instance, you won't be after your 3rd hike of the same place... So simply put, you will get better by training.
BTW, an hour of hiking burns about 500-900 calories depending on the intensity.
cheers
