Not that hard.
Ultima Weapon is not insanely hard, its just a matter of following him until he gets to the canyon. He's really a 50-60 level mob if I remember. The weapons are simple. Once you go through the tedious task of Emerald (with a level 99 party I did it with, never came in risk of dying), Ruby is really a piece of cake with the new All-Summon plus your original KOTRT, and mime materia. The quests dont take long at all, such as the breeding (though it is a thing of luck to get the golden one really, even with the right nut), chocobo races are pretty much simple with a golden chocobo (though all the races can be completed without one), and everything else is usually a "fetch me this, that, and this thing's cousin, and I'll give you a prize" quest.
Not impossible at all. Just a matter of luck/strategy/knowing where to get certain items.
Even so, 24 hours seems freakishly fast for a complete run through of the game (my question still goes unanswered if this was a simple beating of the final boss, or a complete rundown of every single thing).