I'm one of millions of Americans who is over weight and out of shape; but those days are numbered. Being in shape can be very important after a nuclear event. Healthy people heal faster, which will be very important when it may be nearly impossible to go to a doctor for serious illness. Many of the physical things you may be required to do are similar to the things many military personnel have to do: carry an injured person to safety, remove rubble, did latrines, be on guard duty, etc. Plus, once the radiation is at a safe level, you will probably have to plant a far sized garden and really care for it.
With these things in mind, maintaining a healthy body and mind is very important. A few extra pounds won't kill ya, but when you're 10 or more pounds over weight, it make things harder on your body. More important than the weight thing, I think, is being in shape. If you can run a 10-minute mile, walk or jog for an hour or two straight, and do some sort of weight lifting (even if it's low weights, but high reps), I think you will much better prepared for a future with a lot of hard work. Plus, you'll probably feel better overall.