Hey y'all Round Earther Speaking here, I have very little knowledge in the scheme of things but I feel as though this is a valid question.
I've looked through your forums for a while now and I've decided it would be useful to say at the start I'm not accepting "Have you done it yourself" as an answer, I want legit mathematical proof that debunks my question:

"If you walked 10,000 km in a straight line, turned 90 degrees and did the same thing two more times, you would have walked in a triangle. This is impossible on a flat surface."


Edit: Please, if you feel the need to respond, don't slag the wording of my question off, if you legitimately don't understand what the question is asking, just say. Don't give me any comments that aren't answers