Altitude sickness has no corelation to fitness. In fact fitter people can be more susceptable. Hydration is very important as is acclimatisation.
Shouldn' t be any serious effects below 10,000 feet. Above 12,000 feet everyone will feel some sort of symptomns.
Climb high and sleep low, gradually increase your elevation.
I have worked on 1,000 feet increase per day, (Once over 12,000 feet.) and it worked well for me.
I have spent time climbing in the Himalaya mountains, I spent 3 months at 17,000 feet.
When returning to sealevel, I felt like superman.

The worst effected seem to be those who have big goals on limited timeframes.
There is a drug available called Diamox, to releive symptomns, however personally I prefer not to use it, as it can mask what' s happening in your body and you can fail to recognise when things are getting worse.
You can do all the right things and yet still have an off-day, I once suffered pulmonary odemia at 19,500 feet, yet had been fine at 23,000 feet the day before. This was mainly due to a combination of dehydration and severe cold.