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if you can do 300 in 13 hours then you'll likely be trying to sleep around 7pm while it's still light
You have pointed out an obvious point which I had utterly missed :-)
I guess that this means that I need a strategy change. Maybe something like "Have a deent lunch break, then cycle til say 10 and have a kip. I hadn't really thought about congestion at the checkpoints either, I suppose that that is something I need to get my head around. Or go fully independent; I guess a thermorest and a bivi bag or something would be a possible solution
For your 400 it's just a matter of pushing through. If you get really tired, a 15 min nap should be enough to get you restarted.
For a 600, if you can do 300 in 13 hours then you'll likely be trying to sleep around 7pm while it's still light which isn't likely to work. Exploit the daylight hours and try to at least get to 350k or whatever control comes around that. 2-3 hours will be sufficient to get you through the next 250k. Depending on what the organisation is like on your 600, 6 hours sleep may not actually be available. On busier rides then sleeping spaces can get rationed. In a PBP year, most rides will be busy.