I also had a difficult time keeping up with my TPR homework when I took the course last summer. I wasn't in school at the time ( I had just graduated), but since I was working full time it was hard to find time for the extensive homework! You're right, after the class you have to drive home or wherever and then expect to study for a few more hours? Then there's your school homework... I realized that I needed more time to study on my own with the TPR material. I was so burned out from the class that I took most of last fall off from studying.
Most of the other students in my TPR class were still in college at the time. I think they were having trouble also. You have to be really discplined and sacrifice a lot of the things that you enjoy doing, spending time with friends, movies, whatever. But then, what do you sacrifice? You might be volunteering and you still want to keep up with that or shadowing and perhaps you're involved in some clubs and who can forget, you're trying to get those As so you have a very good GPA.
What TPR has to offer and all of the materials they give you are very good tools in helping you prepare for the MCAT. Somehow, I'd make it a goal to finish all of the assigned homework. If you don't do the homework and then take one of the diagnostic tests and you score low...well?
Even though some of the stuff you'll go over will be review, but make sure that you really, truly understand those concepts. Don't bank on memorization and that it'll come to you during test time.
I guess my best advice to you would be to write out a structured schedule of when you'll be studying for school, TPR, where your extra curricular activities will be included, any shadowing, or anything else that is important for your MCAT application. Med schools will see everything that you were involved in, but if you have a low GPA and/or MCAT score they may wonder what you spent most of your time on. Yes, maybe everything else that you are involved in and your personal statement may save you, but I think it is best to have a good handle on everything....sorry for the long message...hope this helps or maybe someone else has some advice for you...