I'm overwhelmed by the distance/time left

Stop.  Take a few deep breaths and get yourself out of autopilot.  Come back into the moment.

Ask: "What do I actually need right now?"

Food? Fluid? A warmer layer? A slower pace? Music/podcast/company?

Decide your next small step. It might be just five minutes, the next loop, or the next landmark.  You don't have to decide the whole race right now