I'd say that the image of californians being healthy and eating health food is more prevalent outside of california than in it. The first time I encountered the idea that californians specifically were different from the rest of the country was when my uncle (who is a health food nut in colorado, by the way) said that californians are like granola: some of this, some of that, a few nuts, and occasionally you get a good bit.
I think a lot of the granola-type healthy image of californians is probably a fallback from hippy times, when it likely was true, at least more than now. I have nothing to back this up, but it seems like since food grows so easily here, it would be a likely place for super-duper-freaky-health-nuts to set up a commune or something and grow all their own vegetarian food. Then, because people tend to remember extremes, the picture stuck.
Now, I'd say that people here are probably more health conscious than people in the rest of the country, but that doesn't mean they do anything about it. Just because they know what all the latest diets are doesn't mean they follow them.
Finally, I'll say that the biggest effect of this image of california being healthy has to do with people who are obsessed with getting a perfect tan, and staying thin so they can show off their tans. People think that a tan is a "healthy glow" and everyone wants a tan (as can be seen by self-tanning lotions). This is probably a result of Hollywood, where it's easy to get such a tan. And besides, the only people I know who *really* care about not having tan lines and such are people from southern california.