This probably isn't a popular stance to make, but I find that most of this stuff with regards to exercise and golf improvement is overstated more often than not. Not that I don't think being in good shape cannot be used to one's advantage, but a lot of the people who think they need to get better hip flexibility in order to improve their hip rotation in the swing need to look at the biomechanics of the body first before they start getting foam rollers and going nuts with hip stretching exercises. I used to do a lot of that stuff as well, only to find that I couldn't get the results I wanted. Then I started to work on my swing mechanics in a better fashion and voila....the hip rotation mechanics improved dramatically.
Now, if you're a Re-Max guy, I would probably recommend hip flexibility exercises. Or if you just have extremely poor hip flexibility. But I honestly believe that most golfers have enough hip flexibility to get the hips to rotate in a fashion that is quite acceptable. They just do not have the swing mechanics that allow for the hips to rotate freely and with ease.
Personally, I think stretching is really good and important to golf, but for different reasons. I think it's key for durability. Golfers tend to get sore lower backs, wrists and shoulders. Stretching helps remedy those ailments. I've got Roger Fredericks DVDs. Very good, but a bit pricey. You could just take a yoga class. Usually some hot girls in those classes, but they add up in price over time. Good way to get out of the house though.
The other part I think is good for golf is cardio. In general, I think if you can keep your heart rate low and blood pressure low, it helps. If I'm playing in Pittsburgh area where it's extremely hilly and I have to walk the course, I'd rather be hitting a ball with my tongue not hanging out of my mouth.
There is another set of exercises I'd like to get into. I won't mention them yet because I haven't tried it. But I think it would work tremendously.
But, I don't really like a lot of the core workouts and weight lifting. If somebody wants to do it for themselves and their life outside of golf...fine. However, I don't think a lot of that stuff will improve a golfer's ballstriking one lick. And I think it can do damage. I don't even think that increasing flexibility and cardio will improve a golfer's swing...but over the long haul I think it helps tremendously.
3JACK
Kettlebell training is very good as well - tough, but it will get you in tremendous shape. Make sure to get a trainer at 1st though would be my advice. A lot of nuances that make huge differences...
