CANADA:
Manpower fit for military service:
males age 16-49: 6,740,490
females age 16-49: 6,580,868 (2005 est.)
UNITED STATES:
Manpower fit for military service:
males age 18-49: 54,609,050
females age 18-49: 54,696,706 (2005 est.)
Add that in to the face that the United States' military budget is 4% of our much larger GDP, whereas Canada's is 1%, then add in the fact that NORAD operations is on our soil, meaning Canada would lose their air defense capabilities as soon as the United States realized what was going on, then add in the fact that Canadian arms are supplied in no small part by American manufacturers. Then finally add in the fact that if Canadian forces moved 100 miles south they would have barely come across a tiny fraction of the US population, whereas if American troops moved north 100 miles 90% of Canada would be behind enemy lines and largely unable to assist. Oh yeah, and Canada would instantly be fighting a two-front war because the Americans could come in through Alaska as well.
Sorry to burst your bubble, but it would be over in a real hurry, even if there were initial successes by the northerners.