A quick suggestion if I may?
Try "running" with them if you ever get the chance to do this again. You might want to consider shooting them from the front. To do this, you can spot one of the runners from afar, When they're still distance away from you, use your zoom lens to point at them as if they're running towards your lens.
Also try a slower shutter speed to portray movement. I understand that you're trying to capture people running, but there'll be even more impact if you an show some background blur (panning) or motion blur on their legs and hands.
Another thing you can try is to shoot the crowd as well, don't just focus on one runner, let us know what's going on behind and how grand the contest is!