Despite today's horrible bombing near the Jerusalem Central Bus Station, as of right now the marathon is going on as planned.
As a public service announcement, the organizers scheduled, amazingly, a minyan for shacharis at Gan Sacher, I think at 6 am. The full marathon begins at 7 am. Gan Sacher is about a 10 minute walk from the marathon starting line.
Someone in my group arranged a shacharis minyan at Chenyon HaLeom, which is right next to the starting point, though about 10 minutes away from the finish line (which is at Gan Sacher). So you have to decide if you want to park closer to the starting line or closer to the finishing line, and then daven in the closer minyan.
So, if you want to daven at Chenyon HaLeom, the minyan will meet at 5:55am at the entrance to the parkign lot and then find an appropriate spot for the services.