I'm also having trouble with the Deep Jungle Cave. Does it have to with which horse you ride. Like bravery?
As the guide says, navigating the cave is tricky. It's similar to the Yeti caves on Glacier. On the first page of the forum ElysiumDream posted a guide (it's a photo) she made that can help you navigate the cave.
And I am horrible at miles and judging distances. I can't tell how long they are! I am having serious trouble! Please help me!
I suggest you use the start pedometer that can be found under "Settings". It counts the distance for you.