Using that long distance runner for even 2 hours would give you 25 miles of travel for a move 5 character 30 for a 6 and 35 for 7 .... and honestly you end up significantly better over land speeds than horses.
Pushing horses in realistic ways as hard as a human might push themselves will kill the horse (we are actually much more tough than we give ourself credit)
Someone able to Run for 24 hours would have 15 times their normal overland travel distance (ok that is not taking a 6/8 hour rest) assuming you do rest call it 10 times your normal overland in emergency you jump off your mount and hoof it yourself.
Unless you have a heroic mount you can empower in some fashion.
Glimli walking is 2.5 mph running call it 12.5 mph (sprinting could be faster jogging less but that is not the description) sustaining that for a full 24 hours ahem that would be insane but going 300 miles in a day pre-automobile that is epic. And even the much more likely 8 hours you are talking 100 miles.
We must save the hobbits.
OK my computations may be using more real world numbers than in game. BUT The descriptions of Gilgameshes ability are that extreme he was epic.
Well, if you want to relate it to actual performance in the real world, just as a starting point for judging the degree of 'epicness' of what PCs are going to do...
7000 picked troops of Gaius Claudius Nero marched from Grumentum to the Metauro, something like 200 miles, although I can't find anything that definitively states the time consumed on the march. We only know that the Battle of the Metaurus took place on 22 June 207BC and that Nero arrived the day before, the 21st, in the evening.
"There was no loitering, no straggling, no halt except while taking food; they marched day and night; they gave to rest hardly enough time for the needs of their bodies."
-- Livy 27.45
where it is made clear that the march was 'several' days. This was also ideal conditions, good roads, supplies pre-cached and people coming out along the way of the march to provide provisions, pack animals, and reinforcements (6000 foot and 1000 cavalry marched, MORE arrived than started).
Anyway, make it 200 miles, it may have been a week, perhaps slightly less. Certainly they made over 30 miles a day, and probably more, as a 20-40 mile march wasn't considered really extraordinary, though a week of them apparently was pretty notable.
Also note the cavalry, which supplied advance forces which went ahead and arranged supplies etc. So we can see that certainly under ideal conditions your horses outdid your men, even over a fair distance (it is said that a Mongol Warrior, with remounts, could cross from Eastern Europe to Karakorum in 2 weeks).
However, horses aren't so good in rough terrain, thick forest, etc. They also need to be rested and supplied with grain or at the very least quality forage (which takes time, they don't eat THAT fast). In difficult conditions you can assume men on foot will beat any animal, except maybe an ass.
I think its safe to assume that tough PCs, traveling roads, can NORMALLY make 30 miles in a day, calling it solid exercise but not enough to be debilitating. Beyond that, or tossing in worse conditions, things start to get into the real serious 'forced march' area. I'm thinking heroes, particularly with training and well shod, can push themselves for several days and probably outdo Nero's legions.