Why were the Misty Mountains so hard to find?

The Misty Mountains were not necessarily hard to find, but rather difficult to traverse.

Now Gandalf led the way. "We must not miss the road, or we shall be done for," he said. "We need food, for one thing, and rest in reasonable safety-also it is very necessary to tackle the Misty Mountains by the proper path, or else you will get lost in them, and have to come back and start at the beginning again (if you ever get back at all).