What are you on? 20km on foot is a great distance, especially when you're unsure of where you are. And 20km in area is a massive amount of space. When you don't know which direction to go, even a mile is daunting.
You're constantly making decisions to travel to avoid trees, difficult elevation, brush which entails changes of direction. The inaccuracies in those decisions can compound. The idea that you can just look at the sun or have nothing but a compass and travel safely and confidently in unknown territory is a myth. It's rarely ever as simple as moving in a straight line.