Totally doable but a LOT of work. Nothing is particularly challenging but the sheer volume of material and new terminology can be overwhelming at times. Science is its own language and you will have to learn it in order to do well both in classes and on the MCAT. Treat it like a job and spend the hours needed to do it right.
khanacademy (referenced above) was a lifesaver for me too btw. Much of the difficulty you encounter will stem from poor teaching/explaining and khan does a great job of teaching the concepts so they become intuitive. It is this intuition that will carry you through the mcat and even through medical school. Thankfully, artists are generally good at building and relying on their intuition, so this may be a strength for you.
I spent ~10 years in the arts with no hard science classes since my sophomore year,....of high school. Now I'm finishing up my first year of med school and have absolutely loved it and find it as stimulating or more so than I found music. I did my post-bacc in a rather compressed way at a small 4-yr college and had no trouble maintaining a 4.0. That isn't to say I didn't spend a lot of time on it -- I treated like a full time job -- but never did I find anything too difficult or confusing that a khanacademy video or 5 minutes with the prof couldn't cure it. MCAT was tough but doable. I took it prior to the post-bacc so I could shave a year off the process and apply while taking the pre-reqs, though in retrospect this probably wasn't the smartest move in the book. I scored in the mid 30s almost solely off khanacademy and some review books.
I've also done very well during my first year of med school although the work load can be overwhelming at times -- just don't let yourself get behind. Ever.
Medicine is an art in many ways and must be learned as any art is learned. You see the same pattern of theory followed by apprenticeship and gradual honing of the craft that you see in how any other art form is taught. I know in the arts there are many times you are faced with a seemingly undoable task: capture the beauty of the scene before you on this canvas, or learn and memorize this 400 pages of opera in a different language and be ready for staging on monday. Artists also learn that despite how overwhelming it may seem, once you put brush to canvas or bang out those first notes on the piano, the rest just starts to flow. I've found medical school to be very similar, always faced with what seems like too much info that's too difficult to understand, and a few weeks later I can't understand why I ever thought it was hard.
I wish you the best of luck -- I firmly believe that medicine could use more artists among its ranks.
