Hi LiH,
Since MBPT isn't around at the moment, I'll throw in my two cents to get the discussion going. I am not a PT b.t.w.
I believe UP is TMS, at least in the great majority of cases. Certain muscles surrounding your pelvis are cramped and painful and pull things out of alignment (not dislocated, which is virtually impossible for the joints in the pelvis). You become asymmetrical as a result. If you look at the list of complaints that are related to UP, you recognize a lot of TMS symptoms.
It might be that when you did your longer than normal walk, your brain sensed danger and set things on fire. I recognize this so well, because I struggled with this for over a year. Being one of my most stubborn symptoms, once and a while it revisits me, but not nearly as severe as it once was and I can always relate it to emotional stress so it passes in a matter of days. I have to add that I am slightly hypermobile, I can overstretch my elbows, knees, rotate my hand 540 degrees, I have flat feet and I have pretty flexible wrists (can pull my thumb to my wrist). I am pretty muscular though, which isn't typical for HM people. One PT that I saw a couple of times gave me inlays for my shoes to stand straight and also tried to correct me like he did with you. The only thing it did for me was hurt like hell and the inlays I threw in the trashcan once I learned about TMS. Maybe I simply didn't trust the guy enough. But anyway, I am pretty symmetrical now.
The problem is that you are probably a lot more hyper mobile than I am (?) plus I am a man. I just don't have the knowledge to say whether you can be so mobile that the pelvis can really get misaligned structurally in a way that it can only be resolved by manual adjustment.