Phantom Steed is an interesting spell. It gives only certain physical stats, and leaves others blank. It specifically describes the creation as
quasi-real, horselike creature. It says it can not fight, and gives exact values for its AC (from which we can determine a dex of 20 or 21), and exact hp (7+1/caster level).
The spell description states exactly what it can carry, and that weight is dependant on the weight of the caster and caster level, suggesting it has no game-defined strength score.
As these values are absolute, you
could claim that the thing is simply a magical creation and thus has no Con score, or you could default to a horse's Con score. It is question for the DM to decide whichever fits his/her campaign the best.
Now here is where it gets fun:
Let's consider what a phantom steed from a 5th level caster can do (the lowest regular level the 3rd level spell can be cast).
Speed = 20*5, or 100 feet in an action. Run at times 4. 400 ft in a round. That's 4000 ft in a minute. That's a mile in 13 rounds.
If you don't consider it to have a Con score, that means in an hour it can cover 45.45 miles. As it can stay around for 5 hours at that level, that's 227.25 miles.
At max speed, (12th level), it's base speed is 12*20 = 240 ft a round. At a run that's 960ft per round. 9600ft per minute, or 1.8 miles a minute. 109 miles an hour. 1,309 miles if you can have it run for the whole 12 hours you have it.
And at 14th level it flies.
Now, some might think this silly. True, but by 9th level the party is teleporting, and by 14th level the party is greater teleporting, so in all it's really not that bad.
If you do assign it a Con score and thus disallow the running for the whole time, the phantom steed still is a good spell. Using the long distance travel speeds from the PHB, at 5th level the horse has an hourly speed of 10 miles at a walk, or 20 miles at a hussle. For the 5 hours you get it, you can cover 50 miles. A light horse needs 8 hours to cover 48 miles.
At 12th level, you have an hourly speed of 24 miles at a walk, or 48 miles at a hussle. For the standard 8 hour day travel day, that's a total of 192 miles. And if you want to push your phantom steed for the whole 12 hours because it's not a real mount, that's 288 miles. And at that level it water walks and ignores terrain effects.
And at 14th level you get that speed while flying.
It might not be
teleport, but it sure beats walking.