I agree 7km are useless - although I keep accidentally getting them.
There's actually a guide on this site that's pretty good, and you agree with it pretty well.
Focus on 5km, while building up 2km and 10km. Burn through the 2km when you get a hatching eggs task (like 5 for a Chansey) or when you have too many of them (only ever need 4, as you'll finish your current egg + 4 for the task). Try to save up 10km eggs to do them all at once with a star piece + lucky egg active for a big boost - or for a double candy on hatch event.