In chapter one we are told that "London, only sixteen miles off, was much beyond [Emma's] daily reach", and that "many a long October and November evening must be struggled through at Hartfield" before Emma sees the John Knightleys and their children. This suggests to me that Emma does not travel to London in the winter, when she might have to return in darkness or bad weather, and that even in the summer months she doesn't visit Isabella more than once or twice a week. Of course, there is no mention of Emma going more than walking distance from Highbury, but --

(from the Morning Chronicle (London) edition 14444, Saturday August 19,1815)
"PEDESTRIANISM -- Thursday morning, Thomas Broughton, upwards of 60 years of age, a tanner residing in Bermondsey-street, undertook, for a wager of 10l. to go on foot from the Stone's-end in the Borough, to Kingston, and back again, in the space of three hours, the whole extent of ground being 23 miles. He started at five o'clock, at which time the odds were against him three to two; he proceeded as far as Wandsworth, where he stopped to rest five or six minutes, and reached Kingston seventeen minutes after six o'clock. After refreshing himself with a glass of rum and water, he set off on his return, in good spirits -- stopped at Putney to change shoes, and arrived at the starting post eight minutes before eight o'clock, completing his wager in two hours and fifty-two minutes. He was immediately put to bed greatly exhausted with the effort. Many bets were depending upon the issue."

While this near-marathon effort is clearly not typical, I think it demonstrates that the tyranny of distance was not an insuperable obstacle to Emma making a day visit to Isabella in the coach, if she wished, with Mr Knightley or merely James and a footman, even quite late in the year.