Judy -- I know I am late to this party, but I also know that you don't need me to remind you of how proud I am of you. I remember a time when you thought you wouldn't be able to get yourself to the start much less the finish. Look how far you have come, sweetie! And it is a hell of a lot further than the thousand + miles you have trained over the past couple of years!
By the way, while everyone here is being so wonderfully thoughtful in helping to think of comebacks to these asinine remarks, can someone help with this? Someone in my office who knows I work out 6-7x/week said a couple of weeks ago "With all the working out you do, I'm surprised you aren't thin. I admire that you still get yourself to the gym every day. If I was working that hard and not getting thin, I don't think I'd bother." Okay, I am sure there is a compliment in there somewhere, but the only thing I could think of to say was "Well, I am getting stronger and I can run a 5k now, so the working out is worth it even if I'm not losing weight." I WISH I could have thought of something really profound to say before I went to the ladies room and cried. I should just remember to come here for validation and not listen to the idiots I encounter and act as if their opinions count. But it is a struggle.
Judy, I am so sorry to

your thread, sweetie!
Cam, I'll give it a try because I'm nothing if not naturally snarky. And there's nothing like hearing people I admire being subjected to mean-spirited commentary that brings out my inner snark. They may not be profound, but I do love a comeback.
I think you're on to something with the idea of a compliment. You might be direct:
"I'm sure there was a compliment somewhere in that comment, because if there wasn't that would make you the biggest jerk (pick your term, and this isn't the first one I thought of using) I've met in a long time."
Of course, you might not want to be so direct as you work with this person. How about:
This one has the benefit of being able to be said in many ways, depending on your situation. You could ask it as a disbelieving, amazed tone, as in "So THAT's why" you don't work out??? Or you could nod sagely, "So THAT's why" you don't work out. Or even half-to-yourself, as if you've figured out something about her: "So THAT's why."
You could do a variation of the above:
"I can see you having that reaction."
Or you could try agreeing with that person. I like the idea of nodding and saying something like,
"Yeah, it can be easy to get so hung up on superficial things like that."
I also like the more pointed,
"Yeah, it must be difficult to understand why I do it if you get hung up on those kinds of superficial reasons."
As for profound, I don't know that I've got any profound statements in response to such barbed commentary. And I'm even more certain that this person doesn't deserve them.
But here's something for you, because you deserve it: "When I think of working out, I think of everything I've gained: fitness and strength, confidence in my growing abilities, good friends, new memories, and even a handful of medals. Focusing only on my weight would dismiss what I'm doing and would limit who am I, don't you think?" And I probably couldn't resist adding, "Plus, it'd be a pretty shallow way of looking at things. That's not me. So I'm so glad you recognize something I also admire about myself." Said with a big smile, of course!
Of course, it's easy to think about these retorts after the fact. I believe I'm just practicing for the next time it happens to me--and it will. There's no shortage of morons out there. (Pssst, I happen to be related to quite a few of them!)