I think it will deepend on what type of exercise you do. Post shtf you will be doing a good deal of manual labor. You should be a leaner version of your curent self, however will you be stronger or have better indurance? Exerting your self and moving will keep you thin, but running for several miles to get help in an emergency or having the power to pick something or some one up that has to be is a matter of training. Thought should still be given to calculated exercise.