JUST DO IT!
OK, this is not a Nike ad, but all work and no play....
You have to do something physical. Play softball, touch football or basketball. If you don't like team sports, play golf, ski, swim or ride a bike. If you don't like sports, go for a walk and lift some light weights. There is something for everyone.
If you need to get started there are DVD's and websites to help. Men's Health is a great place to go for information on working out and nutrition.
A body is like a business, if it suffers from poor management, it shows. If your body is not performing at its best, your mind will not do as well either; and your mind comes in handy sometimes.
To paraphrase Woody Allen, "My brain is my second favorite organ."