There are literally "thousands" of riders on that ride. It will not tend to happen to you. You'll know the riders with your riding level just by the sag stops.

But if you insist, contact the organizer with their online email. They just might have done the mapping with Google Earth. With that, you can commit to memory the last third of the ride. I think you already know that your Garmin Edge 305 does allow all your rides to be viewed on Google Earth.