Many golfers assume that walking and swinging a club for 4 hours is enough exercise. It's great exercise, but it won't help you get stronger. You have to seek out strength exercises to develop muscle and get stronger.
Doing cross back lunges or curtsy lunges, as they are sometimes called, is a great way to develop strength in your glutes, hips and legs.
Stand in a shoulder width stance.
Hold a golf club parallel to the ground.
Cross one leg back behind you to the side as you squat down, keeping your upper body stable. Keep the club parallel to the ground.
Return to the start position and repeat on opposite leg.
Do 8 to 10 repetitions on both legs.
The twisting portion of the exercise will help you train your ability to rotate your torso around fixed hips, similar to the motion needed in the golf swing for a powerful weight shift.