Less than 5 mins, even if you walk really. really. slowly. St Pancras is literally over the other side of the street from Kings Cross, and they are so close they share the same tube station.
St Pancras railway station (/'pæ?kr?s/), also known as London St Pancras or St Pancras International and officially since 2007 as London St Pancras International, is a central London railway terminus on Euston Road in the London Borough of Camden.
Midland spent years borrowing platforms at King's Cross and Euston, before eventually pouncing on some land to build its own terminus. And that's why there are two separate stations. And as to why they were so close, it was a simple matter of land availability in a rapidly-growing metropolis.
The parish was named after Saint Pancras, a 14-year-old boy who had converted to Christianity and would not renounce his faith. As a result, he was beheaded by Diocletian in Rome in 304AD. He is the patron saint of children. St Pancras is a Greek name meaning 'the one that holds everything'.