I worked for about 2.5 years right after undergrad before deciding to go back for graduate education. Then took another 2 years to receive a M.S. degree before DPT school. So in total, about 5 years after post-baccalaureate until DPT school.
I really like that you have a plan in place and have mapped out approximately how long it may take to achieve your goals. You have mentioned taking an interest in health sciences. Do you know that you certainly want to enter PT as a health science career or do you want to just be in the health sciences in general? If the answer is the latter, then maybe shadowing different practitioners is the way to go. If the answer is the former then observe PTs in a variety of settings to really get a sense of how the profession works and where it may be headed. No matter what, ask beaucoup questions, listen excessively, and be attentive/engaged. When I did my observations, I was very limited with what I could do(as most pre-PTs are). A large portion of the time I just listened and watched intensively to stay engaged.
I enjoyed all of my volunteer experience, what little it was. It may range from very low level function in acute care or severe neuropathies to extremely dynamic in a sports medicine/ortho setting. Bring your hard hat and wear it no matter where you are, because ppl will be watching you. I was surprised to see how rewarding inpatient neuro settings can be. I would often sit and talk to the family of patients as they went through treatment. One lady had a moderately severe stroke and her husband was very concerned. I think after about 2-3 weeks, she saw amazing progress and was regaining speech/movement. Good ole neuroplasticity! Her husband started crying to me and shook my hand. Lol! Mind you, I am a pre-PT student, so I quickly directed his handshakes to all of the therapists directly related to her treatment. I may have wheeled her around or stood by her walker but not much more, yet he seemed eternally grateful that the love of his life was on her way back to normal function. You will see/hear stories like this on a daily basis. However, things aren't always roses and kittens, so make sure you pay attention to the "bad stuff" too! Much of that is relative and variable from one therapist/setting to another.
In any event, PT is a wonderful field and I am excited to start DPT school this fall!