Here are some of what I've found to be the best exercises to keep a body in good shape:
Knee bends and squats. A really good squat involves not letting your heels leave the floor. Keep those heels down and lower your butt to the floor and back up again. It feels weird at first, but it really works those muscles. The thing about keeping your heel down is that it keeps pressure off your knees, transferring it to your thighs--which is exactly where you want the tension. Buns and thighs, yaya . . .
Stomach crunches. I've done all of the various, painful, vomit inducing crunches and found a simple one that really works. Hands behind head, raise your forehead to your knees with your butt lifting off the floor. Release, and let your feet tap down quickly before repeating. Do it until you can feel it. The thing about muscles is this: to build muscle the fibers inside need to rip a little, which they then rebuild with amino acids. The pain you feel afterwards is that ripping and rebuilding! It's also the reason many people reach a plateau and need to increase tension to keep building. But, I'm more of a maintainer person, myself.
Weights. I have ten pound and fifteen pound weights that I lift daily to keep my arms toned (though I wish they were a bit more than they are!). If you dangle the weight behind your back and draw it up high--like the Statue of Liberty--you can really help those underarm flabs. Pushups helps too.
If you don't like doing any of these, think about taking a walk every day. Or use something like a Wii Fit. But we--all of us--need to remember how much frequent sitting wrecks our health. There are ways to keep in shape. Remember to drink water, try to eat well, and take vitamins. All of this will not only help your body, but your brain. And a healthy brain means you can give your best to your writing.