Assuming good quality roads without significant elevation changes, healthy horses and well maintained carriage:
- 120 miles in one day, with multiple horse and driver changes. This is pretty much the utter ceiling distance possible.
- 50 miles in one day, but likely not repeatable the next day. (requires a day or 2 rest, or change of horses for next day)
- 30 miles per day for a week, then the horses need extended rest
- 20 miles per day, indefinitely sustainable.
These based upon accounts such as the one by Mr Richard Everard in this thread, London cab records, etc.
Hansom cab horses did about 18 miles per day, indefinitely, moving rather quickly but in relatively short bursts.
Stagecoaches covered up to 60-70 miles per day (more usually half this), but they changed horses frequently, each team only doing 15 miles per day. They also went faster, averaging 5-8mph.
And in detail answer to your question: In ONE hour a 2-horse, 4-person carriage could travel about 15-20 miles. At which point they would then stop, and dispose of the dead horses.