I think chaosmagnet has it right: sit tight and stay warm until you can see where you are going. Even with a GPS or a map and compass you can fall off a cliff, run into a snake or bear, or twist an ankle on uneven ground. Then where would you be? Worse off than before, that's where! Best to hunker down and wait for the fog to clear. This is why shelter and fire/warmth items are so important in your kit. Even when you expect to be back in just a few hours.