Not regularly exercising after I finished high school. You should definitely do that. The time to start really exploring all the cool stuff your body is capable of doing is before and during its peak, not as it slides down the slope into the trough of old age. I find myself wondering how fast I could have run a half marathon 20 years ago if I'd become a runner then, but I'll never know.

I regret not beating out a fellow photographer for the cover of National Geographic, but there isn't much I could do to change that one, it's just one of those "damn, my best wasn't quite good enough" moments. But fortunately photographic ability doesn't decline with age, though the relevance of the medium might.