how much cash?

We carry enough cash for a tank of gas and to eat with. Normally charge gas to VISA for the cash back. This is not only in Canada, but here in the states as well.
When we stop for groceries, we get replacement cash. This way we save foreign transaction fees.
I do not remember seeing any Discovery card logos.

Hi,

I carry a debit card and if I need cash I use it at an ATM to withdraw from my account. I use a visa card for most other purchases with mastercard as a back up. I've been in Florida since November 18 (it is now Feb 10) and I've spent under $100 in cash.

Not every establishment honors Discover so I would take an additional Visa card. Also, we just had a card cancelled due to theft and if that happened to your only card it would be trouble.
We get cash at Walmart when we get groceries. Is there a Canadian Walmart equivalent?

As king27 mentioned, Discovery is not widely accepted in Canada. Make sure to have a Visa or Master Card for fuel, etc.

Northbound I will stop at the first Canadian bank and use their ATM machine to take out $400cdn, which normally will last us till we get to Alaska. Then we do the same on the south bound leg of the trip. On the way back south I will try to use up any of the remains Canadian currency for fuel etc.

Canadian banks and merchants are much better at working with US currency than once back in the US and trying to use Canadian dollars. I think I have about $40 cdn remaining from the last trip and will take that back north the next time

Our last trip was in 2012 so we are a little dated at this time. At that time, almost no one took Discover.

In the early 2000's we used Discover at most locations.

We did a three week trip through Canada last autumn and used a Visa card for most transactions, for the best currency conversion rate, and a debit card for cash withdrawal at ATM machines. I got an unpleasant surprise when I found that my local bank that issued my debit card had blocked all foreign transactions without my knowledge. It took a phone call to my bank to "unblock" foreign transactions for a couple of weeks.
I also was surprised that U.S. currency was accepted, straight up, no conversion, at all businesses. I even paid a parking violation fee to a meter maid in U.S.Dollars.
When I got home I found that a "Red Light" camera in Quebec City had caught me in a traffic violation and accessed a fine of $140 Canadian dollars. I paid this with a Visa card as well.
Every ATM machine I encountered was equiped with the newer technology that reads "chipped" cards. Each machine would state "no microchip detected, would you like to continue this transaction?"
P.S. Canada did away with Pennies a year ago. Another surprise for me.

We carried enough Canadian cash for 1/2 tank of fuel plus some pocket change. Used Visa for virtually all purchases which had no transaction fees and ours had 5% cash back on fuel.