I definitely recommend shadowing a DO, even if you are confident that you want to pursue this route. Initially I was interested because of the philosophy and approach of osteopathic medicine (I got really tired of seeing MD's write Rx's to treat symptoms without addressing the core of the problem) as well as the potential utility of OMM. However, it wasn't until after seeing a DO in action in the ER as well as shadowing a DO FP that I really, "connected" with the field and felt that it is where I belonged.
You need the letter anyways, as it will be a HUGE benefit if you get a good one.
Most people take the MCAT in August of their junior year before applying, or in April of the year they are applying. Taking in in August the year before allows you three opportunities to retake the exam. I waited until August the year of because things came up in April and I had to play the waiting game a lot longer as a result.
Don't bother with review courses such as TPR or Kaplan. It's really hit or miss whether or not you get a useful instructor and any benefit from the course. Instead, save all your textbooks and buy an ExamKrackers study set. This is all you need to do well on the MCAT. You probably don't really even need the textbooks, but they're fun to burn once you get your acceptance letter. I would offer you my EK set if/when/after I'm accepted, but I plan on sacrificing it as a gift to the admission gods.
Give yourself more time than you think you will need. I allowed myself two solid weeks of preperation, as I am a pretty good test taker and have a good memory. However, something drastic came up during that period and I only managed to get in 3 days of studying, so my PS and BS sections suffered tremendously as a result.
Good luck in your endeavor.