Checking with a doctor sounds like sound advice to me... but not all doctors are running friendly. Indeed, I've met several who I could only describe as anti running.
If you really want to stick with running, and figure out what's going on before it gets worse, I'd suggest you make contact with a local running club. Really! Club runners are generally a friendly and helpful bunch, and never tire of talking running, and gear, and different running styles, and, of course, injuries. Clubs often have group runs, and socials after. Many will let you run with them for free a few times before joining. This would be perfect place to talk about your situation and pick experienced brains. Someone might be able to recommend a local doctor/specialist who runs and who understands your specific needs far better than a regular GP could. Someone might be able to help you work on running form - injures are often a result of running style - or at least point you in the right direction. Above all, talking face to face with other runners, many of whom will have had knee issues themselves, will prove far more useful than just taking suggestions online... Of course, that's just my online suggestion.
By the way, addressing the 'pain' before it becomes a full blown injury is unusually smart for a runner!
No, I didn't experience anything like that, fortunately! Sounds like you really hit a wall.
Of course, I had some tough days and weeks, and a few low points, but compared with other people who have walked similar distances I was moving at a relatively sedate pace and not pushing excessively hard.
And also, I was doing what I most wanted to do in all the world. Even when it wasn't easy or comfortable, or even sensible, there was still nowhere else I’d have rather been.
All the challenges and problems, anticipated or otherwise, added value to the adventure. I wonder if you now feel that way about the mental barrier you faced, and overcame? If you had traveled out and back without incident would your journey have meant so much?