I've learned so much.

The first two are both good. They're both grammatically correct, but they have slightly different meanings. "I learned so much from that journey." This one uses the preposition "from" which typically suggests that the thing you learned was something you didn't expect to learn--a moral usually. A moral is like a lesson in life. For example: you were rude to your waiter, so he sent you out without your food. The moral: "Don't be rude to your waiter." "I learned so much over that journey." This one uses the pronoun "over," which suggests normal learning, like you were probably thinking. Maybe you took a trip to a foreign country and you learned many things about the culture. "I learned so much in this journey." Julie is right about this one. It's not correct. "On" would be a better preposition. That would make it closer to the meaning of the sentence with "over," but the subject of what you learned is more vague. It could mean many things. You also used "this" instead of "that" which means the journey isn't over yet.

The first one is perfect. I learned so much over that journey. I'm not entirely sure what thought you're trying to get across to your audience, but something like this perhaps. I learned so much over the length of the journey. I learned so much in this journey. This one just needs a slight tweak, I learned so much ON this journey.