It seems to me all this talk about fast and slow twitch muscle is misdirected insofar as the recruitment of muscle type can't be controlled. What's been left out of this discussion is the most important factor in the acquisition of skill - the principle of specificity - which says that the most efficient way to acquire a skill is to practice that specific skill. Runners should run, swimmers should swim and golfers should swing a club in exactly the way that good technique dictates. Heavy clubs, fans and etc. are not the answer.
First you must have a reasonable goal. Let's say you currently swing at 95 mph and your goal is 110 mph. The solution is this:
Drill a hole through three golf balls and slide them onto a driver shaft about 6 inches up. Drill a small hole and insert a cotter pin so that the balls can't fly off. Now swing at speed and see what your meter reads. If you are swinging at about 110 mph then you are complying with the principle of specificity and your goal. If you can't reach your target speed then take off a ball and try again. These numbers are my experience and I am 71. I am sure anyone reading this can do better. Within days as you gain coordination and strength your speed will go up.
Swing twice a day and no more than three times a week unless you are a good athlete and have great recovery ability. Warmup slowly then go at it full speed for 10 to 15 reps. Rest for 5 minutes and do a second set. If you can't do 10 reps before slowing down then stop because now you are learning to swing slow. When you have increased your speed to about 115 or close to it add another ball to the shaft. If that drops your speed lower than 110 then add half a ball to the shaft. Continue this way until you have added 5 balls to the shaft. At that point your practice club will weigh about the same as you driver. You will have to adjust somewhat because of the difference in swingweight but you will have reached your goal.