You have raised a very relevant question. I wonder myself about these things, so will look for responses/reactions from others.
1. If you need to warm-up (I need to) and stay relatively warm, then you can not get in the queue till 30 min before start. PLUS, you will need a sacrificial set of clothes to keep warm for these 30 minutes. PLUS, you will need to burn some calories while standing in increasingly crowded area.
2. If you start at back, typically you will loose about 1min per 2000 people ahead of you (It is my guess). So you clearly dont want to be at back and loose anything between 5-10 minutes.
3. There will be a few runners in B group who will run faster than 2 hours. But there will be more people in B group slower than 120 minutes (My guess). B group used to be first-come-best dressed (ie no seggregation based on time, I personally think, it is dumb, but C2S organisers will have their reasons). So you will be fine in terms of etiquette balance (read pushing versus pushed). Not too many people give way, you have weave around them (will give your legs a good working).
4. Best place to outrun walkers is obviously the hill and there are few in the first few km. The tunnels etc tends to be a bit of a problem due to overcrowding if you are at the back.
So what are my suggestions:
a) Warm-up/Stretch, keep additonal layer of clothes, get in the queue 45 minutes before start, try to get from side, be prepared to walk a little and run at varying speeds (especially hills).

Use the sidewalk etc to create more space for yourself (risk is twisted ankle).
c) Enjoy from 3km onwards.
d) Be little thickskinned both for people who will be walking in group blocking you and for people who may accidentally elbow you. It happens!
e) If you dont get your target of 120 minutes, there is next year (this race is not for meeting targets at all costs).
f) Have strengths in your legs from 11 km till 14km. It is downhill, you can run freely and end is close.
All the best.
Edited by Super 40, 05 August 2006 - 12:32 PM.