Just some data:
U.S.-Canada border is 8,893 km.
Only 9 countries have event hat much TOTAL land border:
China: 22,147 km
Russia: 20,017 km
Brazil: 14,691 km
India: 14,103 km
United States: 12,034 km
Kazakhstan: 12,012 km
Democratic Republic of the Congo: 10,730 km
Argentina: 9,665 km
Canada: 8,893 km
Of those, these are the ones with a shared border even 1/4th as long as the U.S.-Canada border (2000 for easy rounding):
China-Burma - 2,185 km
China-India - 3,380 km
China-Mongolia - 4,677 km
China-Russia - 3,645 km
Russia-Kazakhstan - 6,846 km
Russia-Mongolia - 3,485 km
Brazil-Bolivia - 3,400 km
Brazil-Venezuela - 2,200 km
India-Bangladesh - 4,053 km
India-Pakistan - 2,912 km
United States-Canada - 8,893 km
United States-Mexico - 3,141 km
Kazakhstan-Uzbekistan - 2,203 km
Democratic Republic of the Congo-Angola - 2,511 km
Democratic Republic of the Congo-Republic of the Congo - 2,410 km
Argentina-Chile - 5,308 km
Other 2,000 km+ borders:
Mali-Mauritania - 2,237 km
Pakistan-Afghanistan - 2,430 km
Colombia-Venezuela - 2,050 km
Laos-Vietnam - 2,130 km
If my math is right, the USSR-China border would have been around 6,500 km.
So there are only two current borders that are even half as long as the U.S.-Canada border (Argentina-Chile and Russia-Kazakhstan). With the India-Bangladesh border coming close to half.