If you had a postillion/post-boy they could also bring the horses back.
The trip from London to Edinburgh - about 400 miles - took 4 days in a post chaise in the late 18th century.
That included stopping at an inn overnight, and stopping for meals and to change horses.
So about 100 miles/day, travelling about 10 hours/day. The roads in England were generally good, by the standards of the time.
A family travelling in their own carriage would probably go slower, so about 2½ to 3 hours for 18 miles.
Most well-conditioned horses can mostly trot for 18 miles, even ridden. It is easier for a horse to pull a vehicle than to be ridden. Heck, I’ve ridden twenty miles in a day myself from time to time. Not, thank God, trotting. There is a reason endurance riders pre-load the morning of a a race with NSAIDS.
In the popular modern sport of endurance riding, a common race length is 50 miles which the top riders finish in about 6 hours; the pace is mostly a strong trot interspersed with cantering. Very little walking!
A heavy carriage horse would go much slower of course but still the normal pace for any driving horse is a brisk jog or trot, not a walk. Indeed horses bred to drive (and they still exist) have conformation selected for maximizing ease of trotting.
Horses in heavy work are not grazed except at night. They can’t get enough calories that way even if they grazed all day. They are “baited” with concentrated food during short rest periods. In Austen’s time that would have been most usually oats; now it’s a high-performance blend of ingredients.