In this situation its hard to give very specific advice... I will try to share a few thoughts.
Are you going to be mobile in a manner that will let you zip to various parts of the run faster than the participants? If so:
- Starting line shots: One wide pic with great DOF, show a sign that displays the reason all these people are there.
- Starting line shots: Many up close shots of the participant's faces, up tight, narrow DOF. Note their expressions... joy? Anticipation? Eagerness? Trepidation? Tell the story of their feelings!
- Mid point in the race: Same recepie... full length body shots of runners struggling and straining. Up close shots, sweat on the foreheads shots from the rear of a group of runners. These events are not always super-competitive, and you can often find 1-2 runners helping another... a potentially very poignant shot.
- End of the race: The finish line before anyone crosses, another of the first person crossing and perhaps a couple others. Don't focus too long on the finish line... follow the first few runners as they have now slowed down and are trying to recouperate. Pics of people bent over, tired, but happy (side or front, NOT from the rear). Pics of people celebrating, happy faces, emotions have to be visible.
You may want to try to find out who are the fastest of the group, focus your shots on them, but do not make it appear as such. Tell their story from start to end in a short 4-5 frames and use other pics to supplement the story.
Afterwards, post process all your own pictures. Your vision of the race may not be the same as theirs. Look at how your shots look in black & white, unless you know that all the pics you take will show in colour.
Be as active and dynamic as the event and it's participants are, and as always...
Enjoy and have fun!