Do they not line you up with faster riders at the front ?
Sadly no, it's whoever gets there first to join the queue although BC riders seem to get preference.
I did the 26 mile route last year and the 52 miles this year and I enjoyed both.
My view is that, apart from the initial scramble away from the line, the roads are wide enough to be able to make yourself enough space to be relatively safe. You just need to stay alert, anticipate what's likely to happen in front of you and keep looking over both shoulders*.
As for the average speed, you'll almost certainly get round at a higher speed than you've previously managed. If you can't make it, I'm sure they'll just (gently) shepherd you off.
You can then rejoin the next lot if you're intent on doing the full route; both years we've used the closed roads to get home, stopping off en route to watch the other riders.
It's quite something to see, especially the families doing the 1 lap route- there are kids on tag-alongs behind their parents or solo on tiny BMXs. It's fantastic.
*Not necessarily at the same time but I'm sure it's a great trick if you can do it.