Well part of her question was about aerobatics. Even basic aerobatic aircraft, such as in my experience Chipmunks and Bulldogs, can a pull a good 4G. 4G is easily enough to grey-out or even black-out someone with lowish fitness levels. If you are fit you could probably sustain 4G all day - but it is still physically strenuous. You don't have to be in an unlimited-class Extra or similar for fitness to make a difference.
Good fitness will also help with oxygen levels, night-vision (as 24C said), alertness, fatigue etc, etc.