The pursuit of distance makes us do damaging things to our swing, mainly because of swinging harder. The surest way to get more distance is not to swing harder, but to hit the ball off the center of the clubface. That takes a lot of practice, which I encourage you to do.
There is another way to get more distance that is easy to do and doesn’t require any changes to your swing. That way is to be relaxed during your swing, thereby increasing your swing speed.
You might have found out that putting lots of strength into your swing also puts tension in your muscles which actually slows down your swing rather than speeding it up.
What I want you to do is take an iron, say a 6-iron or a 5-iron, and make your swing without a ball in front of you. Listen for the sound the whoosh makes as the club goes through the impact area.
That sound is an indicator of how fast the club is traveling. The higher the pitch of the whoosh, the faster the clubhead is moving.
You don’t have to buy any of devices you see on TV that measure your swing speed. The sound of the whoosh will tell you everything you need to know.
Just try a more muscular swing now, to show yourself that it won’t do any good. The pitch will be the same, or even lower.
Even if the pitch does go up, ask yourself what you are doing to clubface control. Odds are you’ve made clean contact more difficult to achieve.
Now set up and relax your grip to just enough that you can swing and keep the club under control. Try this a few times, relaxing your grip more each time, and keep it relaxed throughout the swing.
You might be surprised at how lightly you can hold the club and still control it.
Now relax your arms and swing a few times. Relax your torso. Relax your legs. Get your entire body relaxed, maintaining that relaxation as you swing.
I am sure you will find the pitch of the whoosh rising, proving that you are swinging faster.
More swing speed is generated by more relaxation, not by more effort. Practice this, learn to trust it.