Buddy McGillacuddy
Some days I probably would not mind if someone tried to stick with me for the last couple miles of a hard run on the path. I might even give the guy a high five in the trailhead parking lot. Runners are usually good people.
Other days I am seeking solitude. You are stimuli that I don't need. If running with a friend, you would seem an infringement of our personal space. Glomming off someone's pace without first asking seems either too needy or too aggressive. It's like injecting yourself into a conversation between two people because you overheard something that you made a connection with. That may not always turn out favorably for you.