Or, maybe completely ignore the substance of their comment and launch right in on educating them:
"You know, people llike you approach me all the time. I feel like I have an obligation to alert you--it's really offensive. People like me get frustrated and insulted. I know people like you mean well, but it's really very patronizing. And, you run into one of us every now and then--we get bombarded with comments like yours constantly, so it also gets really old, really fast. It's not polite to approach a total stranger and butt into their personal business, actually--and I can't think of anything more personal than someone'shealth. So in the future it's best to simply smile at someone and keep on about your business.
"I thought I'd let you know, because you seem like a nice person, and I'd hate to have someone like me really let you have it on some day when they can't take any more."
Edited to add:
I'm sure there are other phrases that are less combative:
"Since you wish me well, I thought I'd let you know that the *most* helpful thing is to simply smile and go about your business. I know you want to be encouraging, but the truth is it has the exact opposite effect."