You mention you are life-long US residents - I'm assuming you are US citizens as well. If not, that merits a different look at the circumstances.
If you are US citizens, indeed you do not need a visa to visit Canada for up to 6 months. Upon entry at a land border, the border officer is not likely to stamp your passport (based on the experiences of US friends who drive up from Idaho). However we are now in a digital world so know that your entry has been logged and the 6 month clock starts from that day. From the moment you enter, you have a temporary resident status for 6 months, even if there is no stamp on your passport. It is your responsibility to remember the date you entered.
There is a process for extending your temporary resident status. Google "extending temporary resident status Canada" and you'll find the official word on it. It doesn't say so on the page, but I believe it's recommended to apply 30 days before your status expires. When you apply, the officers will be able to see when you entered so don't worry about that.
Basically, you will have to prove that you have a plan, have the financial means to continue your stay, and have the intention to return to the US, in order for the extension to be granted.
All the best!