This always happens,someone endorses a common sense approach to body exercise and right away that person is attacked for frankly silly and un-realistic reasons.
oops,time for a workout at the ole gym…I will sneak in the back so that my body worship is not noticed…

Please don’t misunderstand or misconstrue me. I myself am an avid exerciser - I was even on the elliptical on the day before I went into labor with my son!
Perhaps I didn’t make my point clearly enough. I did not mean to imply that everyone who is fit has made an idol of their own body - but certainly some people do. I think the temptation to do this can be strong in our culture, especially given its emphasis on youth and outer beauty (the latter often being measured by one’s slimness – at least for women and increasingly for men, too, as evidenced by the rise of male eating disorders).
My point is that it is possible to get carried away and neglect your spiritual life, and in fact, backtrack spiritually. I agree that exercising is something that can be good to do - I myself do it because it makes me feel much better, in terms of giving me more energy, etc. However, I do not accept the blanket statement that it is a sin not to exercise. (Lissa70 makes the argument for why quite nicely, so I won’t repeat her points in my own post.)
Furthermore, I wanted to make the point that spiritual laziness is not necessarily eradicated by exercise. While exercise is one way to introduce discipline into one’s life, if we do not engage in it with the right motives and inclinations, then it will not help our spiritual life.
Lastly, there are many ways to interpret the phrase that the “body is a temple of the Holy Spirit,” and we should not limit our understanding of this phrase in terms of taking care of ourselves physically. We must not interpret this phrase so literally. It has much more to do with not defiling ourselves through sin (which, again, does not necessarily include lack of exercise) and with loving one another and ourselves as co-heirs of God. Additionally, given the kinds of arguments that have been presented in this thread for why lack of exercise is sinful, then many practices which have the potential for physical harm would be sinful. As we see with physical mortification, that is simply not the case.
Sorry this post is so long, but I just wanted to clarify my points lest my words be (mis)used against me again!