Just a few things to say here before this thread drains away. To the 22 year-old who wishes he could do it over again: you are still young, and from a more weathered perspective I will tell you, follow your ambitions now before you're ten years down the road, still wishing the same things, but less able to make the sharp corners in life. Do what you think will make you happy, but whatever you do, remember.. regret is a b-tch.
As for med school requirements, generally one needs a year of Physics, GenChem, Ochem, and Biology to be able to apply. Additionally, many schools may require BioChem, Calc, or some other such thing, making a broad foundation in sciences a good start. Course pre-reqs are entirely dependent on the institution, MS or UG.
Anecdotally, I have never heard of a PA program lasting 4 years. Generally, in my understanding, PA programs tend to be 24-30 months in length with about a year of clinical rotations after didactics. Also anecdotally, PA school and med school are WILDLY incompatible with employment. There is so much information to digest that attempting to work during school could spell disaster. Your mileage may vary.
Volunteer experience in my area is virtually required prior to employment with any paid service. Once hired, however, your volunteering days are over.
What would I do if I could do it all over again? What if the sky was green and the grass was blue? Who f-cking knows. I would probably get derailed in very similar ways. Maybe I wouldn't have spent my twenties at a ski resort, but then I wouldn't have skied so much killer pow. My early thirties seem like a good time to grow up a little. If you're young, have fun.. your body can handle it better. ^_^