Sounds like you've got some promise.... but you must read up on ITB syndrome. Based on what you've said, it sounds likely (though I'm no medic)... and ITB syndrome is something that you should sort out now.
I've never had it, so no expert, but I know you need to do some really specific strengthening exercise... not just rest.... or it will keep coming back and can stop you running for months if it's bad.
Maybe it's not that and maybe it's only a mild case anyway, but you really need to Google it and see if the symptoms fit.
I'm just hoping that they did things properly in the background I don't know the procedures for course amendments, but maybe they got some amendment documentation in the background that adapts the original certification on the website. Hopefully!
I'd say go to a physio as soon as possible. And don't run until you have.
I've not had ITB problems myself so don't feel I can offer more than that... but I understand that this is an issue that is best dealt with proactively - rather than leaving it a week to 'mend itself'.
Final point, hoping you get back up and running quickly, don't get hung up on pace of your long runs. They should be quite a lot slower than HM race pace. maybe 90s per mile slower to help you build endurance and leave you fresh enough to do something sharper in the week.
GPS watches work via satellites to measure distance and in simple terms estimate the distance covered. Because all GPS watches work the same way via the same satellite is why everybody measured the course long.
The second, and more common, type is measurement error: Each time a satellite tries to pick up your location, it can be slightly off. When your watch or phone tries to measure the distance between the last two pings, the line will be longer than what you actually just ran. These tiny errors add up the more miles you go.
You're right in saying that there are plenty of opportunities for GPS watches to measure 'incorrectly' and also for runners to run off the optimum line. But to boldly state that the course distance was accurate is a bit too categorical I think - even if it turns out to be true. Of course, GPS watches are not accurate to the millimetre, and anyone clocking 12.3 or even 12.4m should not jump to any conclusion because that is normal.
But there have been plenty of cases where it's been shown that mistakes in course layout (e.g. not laying out cones to exact follow the route that was measured many months or years previously). So you definitely cannot say that Manchester didn't suffer from that yesterday.
Indeed last year's course was, according to a very good source, probably laid out a bit short. And I mean that came from a very good source! (also I got it short on my watch last year, which I thought was a bit unusual at the time and there were threads on here about it).
If HA found the certification based on last year's course (which was presumably accurately measured, just probably not accurately laid out) - then I trust that the organisers did follow procedure to adjust for the course changes for this year.