Graphic design takes a toll on your body and your mind. Until there are computer implants in our brains, graphic design can involve 40 hours or more of sitting, hunched over and staring at the computer screen. Research is showing that all this inactivity takes years off your life; years that you may not be able to regain even through exercise.
Add to this deadline stress, and the ready availability of snack foods, and it becomes easy to put on the pounds.
I have noticed myself that even a few additional pounds can become a vicious cycle; putting on a few pounds makes you resist excercising, not exercising helps you pack on the pounds, packing on the pounds makes you want to exercise less, until you're practically glued to that chair.
In high school, I was running track and bicycling everywhere and weighed 150. At age 52, I was up to 180 lbs and, though you couldn't tell so much from looking at me, I'd bend down to tie my shoelaces and that roll of flab would jam into my diaphram and I couldn't breathe.
Finally I got a look at myself decided something had to be done. I started with building a deck for a friend and that took off the first 10 lbs. It took me two more years — running and biking during the summer, regressing a few pounds over the winter, and finally biking 190 miles over the 4th of July weekend, running 4-7 miles a day almost every day the third summer, and giving up carbohydrates — to get back to that 150 lbs. (My motivation was vowing to see my abs by the time I was 55.)
I'll tell you that, though I really didn't need to lose all of them, those last few lbs were the worst. Your body gets in such great shape that you really need to work hard to make it happen. (I'm sitting here, having run for an hour and five minutes this morning, and I'm thinking of going out for another run before bed to see if I can at get those last few ounces. Don't get obsessive like I am.)
Take it from a graphic designer who knows how many hours in the chair you have ahead of you — find a sport you ENJOY and do it regularly as a part of your graphic design career. Make it something simple, easy to do (minimum amount of equipment) and inexpensive, so you can't find an excuse not to do it. Running and biking are great for me: I have a great old bike, and I can go for a run in the evening if it gets too late to bike. Doing something that involves other people would be even better; I also do social dancing, and if you can go dancing enough, social dancing (swing, salsa, waltz) exercises the mind as well as the body.
You will find that exercising helps your discipline (I never believed I would run eight miles voluntarily), and getting that blood flowing is great for your mind. Plus, if you have a design problem, you can think about it while you work out.