I have run six marathons and completed three triathalons. Do you think that climbing Mount Kilimanjaro will be a piece of cake for me?
You may not even feel like eating a piece of cake at high altitudes. Your level of fitness has nothing to do with your predisposition to developing altitude sickness. In fact, such a false sense of security and a tendency to bound up the mountain will certainly increase your risk. Your risk of developing the various forms of altitude sickness depends upon several factors, such as the altitude you attain, whether or not you take time to acclimatize before and during your ascent, your rate of ascent (no more than 1,500 feet per day is considered safe), your level of exertion, and how warm and well hydrated you keep. Common sense is much more important than fitness.
