It sounds like you are talking about an event that occurred in the past (going on your trip) after another event (talking about the challenge). Both events occurred in the past, but one was earlier than the other one, so it was "before". "This morning, I brushed my teeth before I ate breakfast." is another example

It has to do with verb tenses. All your verbs need to agree. So even though you're using "before" you're talking about something that happened in the past before something else in the past happened. "Before" is used to create an order of events, and it doesn't have to refer to only things that haven't happened. It is used to compare when two things happened in relation to each other. So the sentence is "before I went" because you're using the past tense already. "We HAD an idea" before "I WENT on" because both verbs are past tense. To make the sentence present tense, you'd change both. "We HAVE an idea" before "I GO on" would be the new verbs.

If it said, "before I go," it implies that the person has not gone on his trip yet. It means that they had the idea, but they have not actually gone yet (perhaps they finished their challenge or are still in the process, but they have not left yet). Example: "Before I go on my trip, I need to pack clothes" implies that I have a trip that I am going to go on (but have not gone on yet), and that I need to pack clothes (implies that I have not yet packed clothes).