I can't say I'm an old hand, but I did it last year when a local charity was looking for riders to fill the spaces they had bought, and asked me for help.
There's a Facebook group full of people worrying about it. With your experience, just go with the flow. Planning to get there on time was the hardest bit - I live out of town and the trains didn't start early enough. I was lucky though and was assigned a fairly late start, and got a lift to join the first train into the Big Smoke.
Expect to spend a fair while queueing, on foot, to get to the start line. Never seen so many bikes in one place. Obviously then, once you're off, riding standards will vary. Watch out for all the usual stuff, plus fast groups coming by at enormous speeds. The stops were well organised with free bars, bananas and so on.