It likely can't dash for 8 hours straight though.  Just as travel pace for PCs isn't the same as combat walking speeds.

PCs move 60 feet/6 seconds = 10 feet/second = 6.8 mph, which is quite a bit larger than the 4 (the speed given for travel at a fast pace).

Both are about 70% faster in combat, while dashing, compared to a fast travel pace.

It likely can because it is a phantom steed, and not an actual living creature that needs rest. Even if it did need rest, an individual phantom steed would only be dashing for an hour before the spell ends.

It likely can't because it uses the riding horse statistics, which is a living creature that needs rest.  Even with rest, dashing for an entire hour straight is not feasible.  It can however go faster than walking speed, but slower than dashing, for a longer duration which is how it getst he fast travel speed.  The same as applies for PCs.

It likely can because, even though it uses the stats for a riding horse, it is a “Large quasi-real horselike creature”, which I interpret as meaning it does not need to eat, breathe, or rest.

Quote from Ovets >>

It likely can't dash for 8 hours straight though.  Just as travel pace for PCs isn't the same as combat walking speeds.

PCs move 60 feet/6 seconds = 10 feet/second = 6.8 mph, which is quite a bit larger than the 4 (the speed given for travel at a fast pace).

Both are about 70% faster in combat, while dashing, compared to a fast travel pace.

 "it has a speed of 100 feet and can travel 10 miles in an hour, or 13 miles at a fast pace" - so it "dashing" long distances only adds 3mph to its speed, not doubling speed for the 1hr it exists. Don't know why you'd ever travel at "normal" speed though - unless you can't ritual cast when going faster (makes some sense - too bouncy), but then just slow down for the last 11mins before switching horses.

5 times max 

Moon Sorcerer also gets this

13mph "at a fast pace?"  That's a ridiculously slow mount.

Didn't think I'd see a Dracula reference here.  "For the dead travel fast."

13mph is over three times faster than the default travel rate. This is slower than a car, sure, but cars haven't been invented yet(if we ignore the hell engines in Avernus) so it's still the fastest way to get around on land. This thing needs no food, no rest and it isn't going to spook since it more of a semi-real magical phenomenon than a creature. 

It can theoretically cover more ground if you make it sprint/dash all the way. But your arse isn't going to survive that; high speed horse riding is very taxing for the rider, and riding non-stop for an hour is going to leave you in no state for dungeonering or dragoning.

Incidentally, this is an is an illusion spell because it falls in with other shadow-like spells that weave physically tangible objects from material from the shadowfell. Creation, shadow blade and illusionary dragon are examples of other spells that do this.