The troll thread where the poster said they tailed someone at a "polite 5 meters" before being cursed at made me laugh but then start wondering what the actual appropriate distances are to maintain when running. Obvious variables - time of day, genders, number of runners on route. Whether using a faster runner to pace, watching a hottie or just absent-mindedly following a leader, I suspect most runners have followed/been followed.
I'd say in a no one else around situation, 50 meters is acceptable. On a popular urban jogging area, 20 is probably sufficient. This is for prolonged maintaining the same pace/distance, clearly you can close the distance to pass someone.
And obviously if the person in front changes their pace, whether speeding up or slowing down, and you change your pace to match, well that's just creepy and inappropriate regardless of anything else.