Learn English – Meaning and usage of the phrase “to be cool with something”

In WWII pilots referred to a routine mission without particular danger—a transport or supply flight, for instance, not a combat flight—as a ‘milk run’, referring to the milk delivery routine which EnglishLearner describes.

This sense of ‘routine’ or ‘unexceptional’ appears to be what’s in play in your example. The ‘milk run’ is a routine group tour where everybody goes to the same places at the same time. This involves tour operators in a minimum of complexity and invention: they’re mostly just doing again what they’ve done many times before, and probably dealing mostly with the same establishments they’ve dealt with many times before.

It is used when you gradually make progress in some field and reach your goals. It is also indirectly implying the hardships that one has to face in order to reach the goal, and restating that there isn't a 'short-cut'.

For example

He started out as a secretary, but worked his way up and eventually became a manager.

Or

In spite of having started his career as a bus conductor, Rajnikanth has worked his way up to become one of the biggest stars in the film industry.

Often it is also used in conjunction with 'ladder'

I'll have to work my way up the corporate ladder.