Instead of the travel time, I think that the matter here is the movement: "into" implies movement, so it can be only used when there's a change in position. And the interchangeability in this case is caused by the fact that, while "in" doesn't imply movement, it doesn't imply its lack either.

Other IE languages also show this sort of grammatical movement marking, although through different ways. For reference, in Latin:

• cecidi in foveam - I fell into a hole (accusative because movement)
• cecidi in foveā - I fell in a hole (dative; I'd interpret it as lack of movement, unlike English "in", but I'm not certain on that)

English also used to have this distinction in the auxiliary verbs that you'd use with the past - "be" if there's movement (even metaphorical), "have" otherwise. You see this for example in Oppenheimer's translation of Bhagavad Gita, "I am become Death" (modern: "I have become Death"), but eventually this usage of "be" was completely replaced with "have". German still does it but... it's complicated since movement itself isn't the sole factor, the main verb also dictates the auxiliary to some degree:

• ich bin in ein Loch gefallen - I have fell into a hole
• ich *habe in ein Loch gefallen - this simply sounds weird