LOL! Probably not.
The border may be long but it’s hardly undefended these days, it’s quite extensively monitored electronically and in the relatively few places where you could get from our road system to the U.S. road system within minutes rather than hours or days, there is saturation surveillance and any illegal entry would be picked up and probably intercepted within minutes. Legal border crossings now have long delays and multiple levels of surveillance — I imagine that agents other than those in the booths are also watching everything and from quite some time before each vehicle reaches the actual border crossing. While this is no guarantee that terrorists won’t cross the border, it has apparently worked. There was one major breach in 2000 when the LAX bombers tried to get in on a ferry that runs from Victoria BC to Seattle. They were stopped in Seattle. It’s not true that any of the 9-11 hijackers came into the U.S. from Canada.
Given the degree of radicalization, I would be more concerned with flights into the U.S. from Europe including the U.K., than with border crossings from Canada. And since we also know this to be the case in Canada, we are just as alert to this risk as your government must be.
I wouldn’t let my guard down, but it does seem that Canada has developed as good an anti-terrorist capability as any western nation. That should inspire some confidence that the border might not be the worst case scenario.