I think that everyone knows about London so I will just add some observations that I was surprised about.

It is big, it is a pretty well-oiled machine, there are crowds of people along almost the whole route cheering and the atmosphere of the other runners was one of camaraderie, not competition (as I’ve experienced in other major races). I found the course not very congested considering how many people were running, which I was impressed about. 

That being said, compared to other races I’ve done I found the pre-race info lacking and the organization not perfect. I’ve preferred other races, such as Paris marathon. There was a lot at the expo that wasn’t explained in the briefings (e.g. where you could get pace wristbands). the finish area was not well marked and when I tried to get away from it I was trapped in a crush of people for several minutes- two runners beside me passed out at this point- no marshals were helping to clear the crowd or direct people and areas were blocked off with no signage. They ran out of some sizes of shirts and shirts were only men’s styles. None of these are deal-breakers but I expected better from London. 

I found the crowds overwhelming at times as the noise was relentless. Most people will love that but if you like to tune into yourself you may be better off picking a smaller or less popular event.