Improving your fitness will directly improve your G-tolerance and your general performance under physical stress.
Actually there's more to that.
With the Dutch aerobatics club I went to the centrifuge / g-awareness training at Soesterberg Airbase which is normally given to military pilots, and one of the messages there was that people with an unhealthy lifestyle resulting in high blood pressure, are actually naturally more g-tolerant than healthy people with low blood pressure.
G-LOC (G-induced Loss Of Consciousness) is caused by fresh, oxygen carrying blood not reaching your brain. Which is simply because your heart is not able to fight the G-forces sufficiently.
A normal, healthy base (diastolic) blood pressure is along the lines of 70 or so mmHg. If you translate that into water pressure you get a blood pressure of about 1 mH2O - the equivalent pressure of a column of water 1 meter high. But under 4G, this blood pressure is only able to lift about 25 cm of water. Which is more or less the vertical distance from your heart to your brain. And thus the reason that normal, healthy people have a normal base G-tolerance of about 4G. Meaning that they can deal with 4G without any training and without any ill effects.
In order to become tolerant to higher G levels you either need to reduce the distance from your heart to your brain or increase blood pressure.
For some people this happens naturally. People with short necks or people with naturally high blood pressure (usually the result of an unhealthy lifestyle) are naturally more G tolerant. For the rest of us, it's a combination of:
- A reclined position (think F-16) - this adds about 1G to your tolerance
- G-suits, which can add about 2G to your tolerance
- Contracting strategic muscles and a special breathing technique to temporarily increase blood pressure in/to your brain. This can actually, if applied properly, consistently and continuously, add at least 3G to your tolerance.
Needless to say, if you're healthy and properly trained it's easier to apply the straining/breathing technique over a longer period of time. So don't read this as an excuse to live an unhealty lifestyle!