Regarding the pen situation, I did the race last year and this was the one area that let it down. It was all a bit of a bun-fight to get in the start area, surrouding area was too small and congested, in the end I scaled a security fence to get in, bit risky just before the start of a marathon.
We set off in 2 waves about 5-10 mins apart. Your wave is set by your predicted time on entry.
Other than that brilliant event in a great city. We had lovely spring sunshine which helped, hope your all as lucky and have a great time