I've only done one official one since too.
It was poorly organised in general, we ended up starting at the back because there was no way of getting further forward. I was still overtaking people at the end (humblebrag or what)...then at the end, there were people stood in the way of the finish line with prams etc. It was just completely shite all-round.

Another reason is that I've found the fella that runs BTR to be a bit of a knobhead, over the years