The first usage that I can find that refers to Israel as a "land flowing with milk and honey" in Exodus 3:8:
וָאֵרֵ֞ד לְהַצִּיל֣וֹ ׀ מִיַּ֣ד מִצְרַ֗יִם וּֽלְהַעֲלֹתוֹ֮ מִן־הָאָ֣רֶץ הַהִוא֒ אֶל־אֶ֤רֶץ טוֹבָה֙ וּרְחָבָ֔ה אֶל־אֶ֛רֶץ זָבַ֥ת חָלָ֖ב וּדְבָ֑שׁ אֶל־מְק֤וֹם הַֽכְּנַעֲנִי֙ וְהַ֣חִתִּ֔י וְהָֽאֱמֹרִי֙ וְהַפְּרִזִּ֔י וְהַחִוִּ֖י וְהַיְבוּסִֽי׃
I came down to deliver them out of the hand of the Egyptians, and to bring them up out of that land unto a good wide land, unto a land flowing with milk and honey; unto the place of the Canaanite, and the Hittite, and the Amorite, and the Perizzite, and the Hivite, and the Jebusite.
The commentaries that I saw, Seforno and Ramba"n on this verse described this metaphorically. Ramba"n, particularly, relates all the other terms ("good" and "wide") towards describing the "milk and honey" concept. I.e. - the good and wide land has fresh air which is good for the animals to make lots of milk. Also, that the fruits are fat and sweet until the honey flows out from them.
If I recall correctly, some place in the Talmud, this pharse is taken literally, i.e., that there was a flow of milk and honey. I don't recall where this is.
If this is to be a metaphor, why does the Torah use this metaphor rather than stating more directly, something like "a land with good air for the animals and sweet fat fruits" or something similar that's not metaphoric?
Lastly, why focus on these two qualities in particular? The land has many other fine qualities. (Yes, in Ekev it talks about 7 species, iron and copper. But the more dominant phrase that we see is "a land flowing with milk and honey".)