Jumping distance takes current speed minus whatever is considered normal speed and then adds the value of "JUMPING". So, the game already processes that for you.
Played around with this a bit, and came to some way of implementing the feature. Mind you, this is a really amateur fix; there's probably a much better way going about solving this.
1.) Open Part 2 of BlizzABS. Locate 'def update_control' (Line 3592). A few lines below, find 'player.move_speed = player.normal_speed' (Line 3596) and replace it with
player.move_speed = player.normal_speed unless $game_switches[id]
Replace 'id' with whatever switch your horse event activates on.
Go down a bit and find 'player.move_speed = Config::RUN_SPEED' (Line 3605) and replace it with
player.move_speed = Config::RUN_SPEED unless $game_switches[id]
Again, replacing 'id' with the switch you want.
2.) Open Database and go to Common Events. Make it Parallel Process with the condition switch the same ID as what you just put above. Insert Script:
if Input.press?(Input::Run)
$game_player.move_speed = 6
else
$game_player.move_speed = 4
end
Change the move speed values to whatever you want.
Worked for me. Like I said, very crude fix...