I found London so deeply irritating that I would never do it again.
People weave about in front of you, slow down in front of you, STOP in front of you, and generally make it a very frustrating event. I had nothing to compare it to, as it was my first marathon, but it certainly slowed me down (from a not very fast pace to start with), as I kept having to change direction to go round people.
It was worst past about eighteen miles, when everyone is tiring.
As cougie says, though - your long training runs should be slower - quite a bit slower - than your intended race pace.