Just walk? 250 miles from Albany (the northernmost depot) to Montreal.
A months hard marching.
Requiring (if "just walking") 100,440 wagons to supply 100,000 men at that distance.
"Just walking" isn't an option.
Chicago is around by this time, so is Cleveland, Pittsburgh, Cinncinatti and a host of other cities, ALL connected by rail. It is 1860 not 1820 so there is a lot of US cities around with lots of railroads. The US has plenty of experience building them and can add to it easily. By 1860, any time the US wants Canada it gets it. There are far too few Canadians to defend it and GB is too far away.