Dear joz,
Not to minimize your concern, but I don't think you have anything to worry about with regards to your undergraduate academic performance. Your GPA is right at/slightly higher than the average for most MBA programs. While your institutions might not have been the most selective, that does not make your education less than or your performance seen as below average. There is no reason to explain your choice of schools in this regard. If you do score 680+ on the GMAT, then it will further confirm your academic aptitude. Assuming you do score high, I think you have the foundation for a competitive application.
Best of luck,
Lisa