A hook will always go further than a slice. Learn how to hook the ball by correcting this major swing flaw, and I guarantee you will add yards right away. That is just one example, but the point is to look at ways to improve your golf swing first. If you are able to strike the ball more solidly, you will see an immediate increase in your driving distance!
Say you do actually have a decent golf swing, but you want to add some yards. What should you do now? Join a gym with the goal of being able to bench press 300 pounds? No, absolutely not. The best way to increase your distance off the tee, if you have a decent swing already, is to improve your flexibility, especially with your shoulders and your trunk. This will allow you to make a fuller shoulder turn. A fuller shoulder turn will help you to widen the arc of your swing, a key component to hitting it long.
This is especially true with older players. As we watch our golf heroes age, they Bench Swing Guys flexibility and their swings get shorter and shorter. Arnold Palmer is a clear example of this. If he had focused a bit more on his fitness and flexibility at a younger age, he could have a longer swing than he has now. We can't eliminate all of the issues that come with aging, but we can sure hold them off as long as we can.
With that in mind, get yourself on a stretching program. You can learn a few key stretching exercises that you can do from home in 15 to 20 minutes. Stretching should be done every other day. The hamstrings, back and shoulders are what require the most attention.