My experiences were from 4 years ago as well. I included some activities in high school on my AMCAS, but most in college. I didn't know about the "post-secondary" rule, but I do not have the same experiences that an average pre-medical student has because I only decided on medicine as a career path a year ago.
I wondered too, whether it was short-sighted, to apply after only a year of coursework and orienting myself to the entire process. However, I went with it. In high school I volunteered in the Respiratory Care department at a local hospital 4 days a week for 3 years and thought at the time that I would go to school to take pre-reqs and transfer to an upper-division transfer college for Respiratory therapy. I took these pre-reqs as a nursing major for hospital and clinical skills experience, and switched as a nursing major all together. But after awhile, I decided that what I liked was the biology behind human disease. I began to do research and was truly interested in the science behind medicine. Then I was hit with a dilemma - apply to med school or grad school? I wanted to work directly with patients, but still had a passion for research. So I applied MD-PhD. Am I inexperienced ? Probably, but I know that I fully explored other options in health care before making my decision.
I would list past activities, because the decision to go into medicine and the experiences along the way shape you. Don't look back, look forward and be happy for what you have seen and done along the way. Good luck