Do any of the people who ask you this have a habit or hobby that they enjoy? If so, the next time one of them asks, "Why do you work out so much?" just reply with a question of your own, "Why do you scrapbook (or whatever) so much?" If they're smart, they'll get the answer right away: They scrapbook because they enjoy it and they feel a great reward from doing it. Ah-ha!!! That's why you work out, too, isn't it? (Well, it's probably one of the many awesome reasons to work out).
I also like the answer, "Because it helps me manage stress," as well as the answer "I want to be healthy". And for those days when I'm feelng cantankerous and impatient, a nice "What do you care?" would fit the bill nicely. Of course, this is only to be used sparingly and only when the person who's asking is particularly rude or has harped and harped on you about your workout habits before.
As far as someone speaking up and saying, "Oh, she won't do that...she won't eat such-and-such.": I think that would happen only once, and then I'd politely but directly tell this person that I am not a two-year-old and that I am quite capable of speaking for myself and making my own decisions. I don't need them speaking for me.
You know, I don't get people making these statements to me, anymore, but back when I was into bodybuilding, people (mostly women -- jealous maybe?) would make little snide comments about it. One such comment was, "You don't need to work out, you look just fine." and I'd reply, "Well, how do you think I got this way?" Duh! I looked "fine" because I worked my a$$ off and hit the weights for 1-2 hours a night. I mean, there was a lot of the proverbial blood, sweat, and tears that went into making that body. I sacrificed a lot to look that good. Unfortunately, naysayers refuse to see all that and would rather rip you apart and bring you down. I distanced myself from those types of people, because they're not the kind of people worth being around, IMO. Petty, superficial folks! If they can't see beneath the surface, then be off with them!
But feel free to use the "How do you think I got this way?" response when the opportunity presents itself to do so. It shuts them up pretty quickly.