I've tried SSRIs while not depressed and noticed mind-altering effects and physical side-effects immediately. This idea that SSRIs don't do anything at all until several weeks in just isn't true, even though that's how it works for most clinically depressed people.

Who said anything about not doing anything? You'll notice the most side effects in the first week, including mind altering ones. The most potentially beneficial effects of those that occur immediately can be expected to be in reducing any obsessive negative thoughts. Rather it is the symptoms of depression that start to alter only over the longer time scale.

Any specific reason why it was more likely to be a rare effect of prozac than the placebo effect or simultaneous changes in diet or exercise?