You discover more road by walking than by studying the entrance.

Studying the entrance gives you information about the entrance. Everything past it is only visible from further in, which is why the route always looks shorter and vaguer than it turns out to be, and why no amount of advance research substitutes for the first stretch of walking.

Stop trying to see the whole path before committing. Go far enough in that the next section becomes visible, then repeat. That is how routes are actually discovered.