Generally, this term is used by those who need to release urine. The symbolism of "long horse" and "short walk" are up to the user's interpretation.
John: You know, though our conversation is enthralling, I admit I must taking the long horse for a short walk. In other words, I must urinate, as you may know from the saying's entry on Urban Dictionary.
Jane: Ok.
12 step recovery jargon referring to someone new who talks about how great life is, now that they'resober. Usually meaning that the person is out of touch with reality.
The new guy seems pretty happy for a dude who has no job, no money and no family. He must be on a pink cloud.