Good thread.
A couple of years ago, I took a year away from playing golf specifically to focus on fitness - cardio, strength, flexibility. I joined a gym, and since then, I've gained 10 lbs of muscle, am stronger than I ever was, trained for and ran a 10k, and finally was able to touch my toes with locked knees. All of this was primarily with better golf in mind, but the residual effects of a better life are immeasurable. Oh, and I am always tinkering with my diet, now incorporating more veg, less meat/starch-- still carrying around too much body fat. I also don't neglect proper rest/recovery time so as not to exhaust myself.
Now I'm back playing golf, and the difference is noticeable. Injury free, zero back issues, better swing, no fatigue.
All that said, now whether I'm golfing or not, I'm keeping the fitness routines/diet/rest in place forever. And hopefully the golf, too.