Quote
The vans were similar to the stage-coaches, but much larger and clumsier, and jogged along at a very easy pace. They took, in fact, from fifteen to sixteen hours to perform the journey under the most favourable circumstances […] One van left Portsmouth at four p.m. every day for the “Eagle,” City Road, London, arriving there at about seven or eight o’clock the next morning, and another left the “Eagle” for Portsmouth at the same time. […] Thirty-five years earlier (circa 1770), even the quickest stages were no speedier than the vans. For instance, at that time the “Royal Mail” started daily from the “Blue Posts” at two p.m., and only arrived in London at six o’clock the next morning.”
— The Portsmouth Road and Its Tributaries, Charles George Harper, 1895 (source)
Of course, if you really put your foot down you can do it in ten.
And if you’re king - well, in 1773, George III did it in 6h45m.