If you're the 'designated photographer' (even if unpaid), you should have the opportunity to shoot at the starting line and the finish line, and possibly in between (I presume you'll have access to a car, and know the marathon route). Since the runners will be bunched at the start, consider standing a ways back from the starting line with a telezoom to get a nice perspective compression of the field of runners. If there's a starter pistol shot, try to frame with that in the shot and fire as it goes off, you'll likely catch the puff of smoke. Then move to the side with a wide/ultrawide and capture the group with a wide lens as the runners as the go by, possibly from a low angle.
If there's a spot on the course where there's tough uphill climb followed by flat or better yet slightly downhill, you can position yourself a short distance after the hill with a telezoom, catching facial expressions as they crest the hill, then full-body shots as they increase pace toward you (servo tracking with the 5DIII). For body shots, try shutter speeds fast enough to freeze motion, but also slightly slower so face/torso are frozen but you capture motion in the arms and legs.
At the finish line, catch the winner breaking the ribbon (duh), and other runners as they cross. Helps to have other photogs so some can catch the winner and early finishers cooling down, while later runners crossing the finish line can still be captured.