Originally Posted by DrIsotope

You're going to want to have an external battery hooked up, for sure. An Edge 820 with nav on might get you 6 hours. So here's my recommendation. The night before, attach a micro USB cable, and blob all around the port with this:

Even easier to waterproof the battery. Just put it in a baggy, or run a cable long enough to put it in a saddlebag or similar.

After the ride, the liquid electrical tape can just be peeled off.