Ships travel non-stop, riders pause to water themselves and the horse,
eat, sleep, and the terrain goes up and down, and forces them to wind
around things. Plus, they ride slowly--at a walk--most of the time.
Horses really are not designed for long range running, particularly with
a man and gear loading it down.
A horse at a walk is marginally faster than a man. But military history
also shows that men are willing/able to be forced to travel longer than a
rider realistically can. As a result, infantry had to push to keep up,
but not that hard. Cavalry is fast and mobile _on the field_ where it's
relatively short dashes from place to place.
I think that flight is not faster because our fliers are having to _work_
at it, not glide and soar all the time. As a result (and the rule that
you can't fly over water for longer than so long supports this), fliers
have to land to rest and feed. Your advantage is that you ignore
terrain, and you move fast. The disadvantage is that you can't move
constantly, but have to move in hops. Not unlike having a 2 gallon gas
tank, and travelling long distance--if you check at the end of the
journey, the time spent fueling is a _big_ chunk of the journey.