Looking for a short story. A professional runner notices an unfamiliar face at the track. The newcomer keeps pace with him, then eventually beats him. Later he is running outdoors and is transported to another planet/dimension. The stranger he saw earlier tells him he is being hunted and gives him a weapon and a head start.
Came across this finally, I had some details off but it was "Stride" by Robert Reed.
Originally published in Aasimov's Science fiction (Asimov's Science Fiction, November 1994) and collected in The Dragons of Springplace;