How did grain get to Karakorum in the 13th century?

Some time ago I read Devereaux's post: https://acoup.blog/2019/10/04/collections-the-preposterous-logistics-of-the-loot-train-battle-game-of-thrones-s7e4/

the fundamental logistical problem of the pre-railroad era: everything you use to move food that is not a boat, eats the food, and does so quite quickly. Armies were thus restricted to sticking to areas where their food could be gathered locally, or sourced by sea or river transport (the latter way way more efficient – a single ancient grain freighter from the Roman period might bring in several hundred tons of grain in a single run).

I want to be clear here: the logistics burden that makes this impossible is not the infantry, but rather all of the horses pulling the wagons full of grain that – from a logistics standpoint – is useless because it needs to reach the end-point uneaten. Those horses are the massive logistics burden that breaks this plan.

I was surprised, then, to find this note in The Pursuit of Power, McNeil (1982):

Under Kublai and his successor, for example, the garrison of Karakorum received more than half a million bushels of grain each year from China, deliver by wagons that took four months to make the round trip.

p57

How did the Mongols make this practical?