and if we assume it's the first Doctor - bizarre as the thought of that incarnation walking across China is
He travelled a fair distance across China in "Marco Polo," though I don't think he was on foot for most of it.
And I daresay there are a lot of people in Vietnam who would have a different perspective on whether Nixon's policies (among other presidents') resulted in mass murder.
The Doctor very quickly whines his way into getting himself carried in a wagon or litter. When he's forced to use a horse, he makes such a fuss about his arthritis that he instantly wins the sympathy of fellow sufferer Kublai Khan.
Yes, I did think about mentioning Nixon and Vietnam, but decided it might lead into Neutral Zone discussions. Thinking about the wider point of the Doctor's dodgy acquaintances, he does have a habit of helping people just because they're on the run from nasty guards, and then leaving them in charge without wondering how they'll handle power. Peter Purves, when he started going to conventions, said he wouldn't mind coming back for a story where the Doctor revisits the planet where Steven was left, only to find he's become a worse tyrant than the people he replaced...