In reply to GHawksworth:
Obviously you need to maintain your level of fitness, but altitude is funny. Sometimes you ar just fine with it, other times it's shit.There is no way of anticipating how your body will react, and it can vary from trip t trip. Sometimes it's the fittest who get worst affected, sometimes not.
A good long walk in with ever increasing altitude seems a good way to acclimatise, but pace yourself and don't rush things. If time allows, having gained altitude, drop down again to a lower altitude to sleep, before going back up again.