5 years is kind of the norm nowadays, especially considering some pre-meds with majors outside of the sciences already take 4 years for their courses and then they have to add that extra year of pre-reqs. For example, my school only has a 4 year graduation rate around 20%, with the majority of students graduating in 5-7 years. I am going in to my 5th year as well, I took a full load every quarter (12-17 units), and every summer, and I still need my 5th year to finish.
Regarding retaking your pre-reqs, it has been advised by many people on SDN to avoid retaking any classes you have already passed (C or higher). Retaking them would mean that you are expected to get an A the second time around, and you risk it negatively impacting you if you do not get that A. If the class was a series class and you showed improvement in the next class, that will show them that you figured out what went wrong the first time around and that you worked to improve it.
You could retake some classes you got C's in if you have a lot of them, but having 1 or 2 shouldn't be a problem.
Also, taking upper division science courses may look nice, but you want to make sure you will be able to do well in them. You don't want to have a decrease in GPA your last year, it may come off as laziness or an inability to do well in more challenging classes.
