Are you after shots of individual runners or more atmospheric shots?
If you want single runners find a spot 1 mile or so in where the pack has spread out and are running towards you but will turn away. This will get you a clear shot of the runners coming straight towards you before they turn away leaving the next in the frame.
Either lens at the long end will do, which one depends how close you are to the runners. I normally use a 135mm but my 300mm is just as effective at longer ranges. Focus speed in servo mode is the main thing to worry about.