The eggs that you obtain from the game have 5 different distances that you have to perform by walking around outside. These Pokemon eggs contain a large amount of candy once a Pokemon has been hatched from the egg.
There are 3 kinds of eggs that you could get your hands on rather quickly by pokestops for example or by obtaining it from a gift from one of your friends. Also, with the weekly Adventure Sync reward, you could obtain an additional egg if you walked at least 25 km a week.
The Strange 12-kilometre eggs are only obtainable from beating Team Go Rocket leaders. These will end up in your bonus storage section and i would bet that the same will be used if new eggs are added in the near future.