Flexibility and strength
The need for flexibility becomes obvious when you look at the nuances of the golf swing. The hip turn, the shoulder turn and the arm extension come much more easily when the golfer is flexible. Golfers need to stretch out their lower back and their hamstrings. A good lower back exercise is to get on all fours and raise the lower back up to a count of four and lower it for the same amount. Stretch out your hamstring the same way a track athlete would, with one leg bent behind you and the other our front, bringing your hands to the outstretched leg. Hold this for a count of four and then do the same with the other leg.
Strength is also important, but perhaps not quite as vital as flexibility. A stronger golfer will hit the ball further, as long as the swing is flawless. However, a stronger golfer may have a tendency to swing harder, which is not advisable. Circuit training on Nautilus machines and training with free weights brings best results.
A golfer needs endurance--both of the mental and physical variety. While walking 18 holes for 4 days is not the equivalent of running a marathon, a golfer who has endurance will not get tired and will not take shortcuts in his game. Running 2 to 3 miles two or three times per week should be enough to get you through four rounds under difficult weather conditions without any trouble.
Mental endurance has to do with visualization and learning how to get past hurdles. This is done by understanding your goals and knowing what it takes to execute each shot successfully. Golfers must realize they are competing with themselves and not the other golfers. Don't worry about what the other guy does. Just concern yourself with your own shot and execute it as perfectly as you can.