According to google maps, you're looking at 78 hours of driving:
http://goo.gl/maps/rnyYy
That's a lot of driving, especially over 16/17 days. If you think about it, spending 12 hours a day in a car is the most that I could reasonably tolerate; here you'd have to do that over 7 to 8 days just to cover all of the ground with no stops. With two days at Mount Rushmore, this leaves only 6 to 8 days to actually spend time in the other cities that you're visiting.
If you can do it, more power to you (perhaps I'm just jealous because I love road trips myself)
EDIT: If your GF doesn't like flying, another option is taking the train. Amtrak goes from Portland/Seattle through Minneapolis and into Chicago. From there you should be able to get to Buffalo and up to Toronto. Maybe another option to consider now or in the future...