God only knows what possessed me, but I went looking and found that the general consensus of fans is that the fictional Riverdale is on the west side of the Tappan Zee bridge over the Hudson River, in Rockland County (which apparently gets mentioned in the show, and there is only one Rockland County in the United States).
So, assuming that's correct as can be for a place that doesn't really exist, it's 33 miles from midtown Manhattan and 485 miles from Quebec.
More I-can't-believe-I-bothered-knowledge: The show is actually shot in British Columbia.