The renowned British mountaineer Don Whillans, whose round physique betrayed a fondness for pints, put off training for Himalayan peaks till the approach trek. I would not recommend this strategy.
Physical fitness is an important ingredient for an enjoyable climb.
A program of moderate exercise, starting at least one month before departure, is highly recommended. Aerobic exercises like running and biking are fine, but hiking uphill with a pack is best. If you are tackling one of the serious mountains around the world it is advised that you mix in some light weightlifting or rock climbing for overall fitness. Remember, in climbing the pace is relatively slow, but the duration of activity is long. Get the scoop on staying healthy up high from GORP's Expert Hiker, Karen Berger.