Don’t get too hung up on horses. While the rulebooks have all sorts of rules for movement speeds while on foot, encumbered, or riding, the truth is that all travel takes exactly as long as the DM wants it to. This brings to mind the wise old saying:
"A player is never late, Dave. Nor is he early. He arrives precisly when the plot dictates he should."
Or something like that.
Perhaps games like AD&D are best explained as games of solitaire, where the DM is the player and the players are his cards.
* Note that I'm saying that linking Shamus's work is passe for me. Actually reading it is not passe at all!