He determined to enter Tibet from the north, and started from Peking on December 17. The choice of this route made it necessary for him to make in the first instance for Lan-chou, the capital of the province of Kan-su ; so he arranged with a cart firm to supply him with two carts, with two mules each, to take him there in thirty-four days.[...]
From Lan-chou, Mr. Rockhill advanced to Hsi-ning, the residence of the Chinese Imperial Controller-general of the Koko-nor, an official known to the Tibetans and Mongols as the Amban.
Source: wiktionary
Soon after leaving the village we crossed some rising ground, meeting on the summit a caravan of forty mules laden with drugs from Lan-chou, the capital of the province of Kansu. These drugs are sent down by cart to the department city of Ch'in Chou, eight stages from the capital, and there transferred to and thence brought down by pack animals.
Source: wiktionary
There is nothing implausible about this, but there will certainly have been cities along the way, most obviously Lan-chou on the Huang-ho River at the southern end of the Kansu corridor, which, assuming that Pegolotti came that way, he would have found impossible to miss.
Source: wiktionary