from London to Kingston, and no change of horses.

That is according to the 1810 edition of John Cary's New Itinerary. Of course, the stage coaches worked the horses into the ground, and one probably wouldn't treat ones own horses (and especially not Mr Woodhouse's horses)that way. Also, the Woodhouse stables might be full already, thus obliging the Knightleys to change horses half way in order to send their own horses back to their own stable.

I think Mr Woodhouse is the limiting factor - Mr Weston can go to London in March and come over to Mr Woodhouse's after dinner in the evening, and he had only been "been in motion since eight o'clock in the morning"(35), getting to London in business hours (more likely banking business hours of 10-4 than 9-5, but certainly not the working class factory hours 7-9!) The sun would be rising around 6:30am and setting just shy of 6pm, in March, and Mr Weston can make the trip to London and back to visit Mr Woodhouse that night without distressing him. Of course if Mr Woodhouse's fears did not have to be considered, John Knightley might commute if he liked (and if his family were at Hartfield, he might not be so amazed at such a venture.)