I must admit, if I was on a run with a buddy and some random guy decided to speed up and latch on to the back of us without saying anything, I would consider him some combination of joker/creeper as well.
That's like having lunch or a coffee with a friend at a 4 person table and some random guy decides to sit at one of the empty seats without saying anything because they were curious what kind of coffee you were drinking. That's weird and unwelcomed regardless of whether he "beared no ill will" or weren't "disrupting our lunch in the slightest."
This. Even worse is the fact that the guy wouldn't take the hint when they tried to drop him. This is akin to the stranger at the coffee shop following you when you get up and move to a different table to have some privacy.
It would be one thing to walk into a coffee shop, see a group of guys who seemed to have a common interest and say "hey, I noticed that you guys have X university track and field gear on. I ran for Y university. If you're not doing anything important, mind if I join you?" But if they say no, it's time to buzz off. Similarly, you can ask to join another runner, but if they aren't interested, don't just insist upon running with them.