It works like most humor works: it sets up an expectation and then defies it. The expectation is that they see him charging combatively from a mile away. They s

It works like most humor works: it sets up an expectation and then defies it. The expectation is that they see him charging combatively from a mile away. They should be readied for an attack or at least an urgent situation. This is defied by him popping up almost immediately. It's also defied in very "British humor" style, with the characters seriously under-reacting to the situation. Under-reacting to the approaching threat (which conveniently also serves to highlight the distance) and to the attack itself.