I ended up riding this route on Thursday as the weather looked terrible today.
The route itself was great - mostly smaller roads, never on big roads for long, enough interest to keep the motivation up. So, that aspect of it I was really happy with. Some nice villages, Humber Bridge, Lincolnshire Wolds, lots of combines out harvesting.
It was my first 400, and given the experience, possibly my last! I just found it so exhausting, particularly in the latter stages once darkness had fallen. I felt the need to rest a lot and even had a short lie down in a bus stop, but wasn't able to sleep which is what I really needed, even just 5 minutes.
So it was a case of deciding to stop for a bit and feel a bit better for a short time or press on and get back sooner.
I rode it solo so my safety was more of a concern than if I'd had company. I felt that I was walking (riding?) the tightrope between being alert enough (pro plus, coke and coffee) to overcome my inner exhaustion and ride safely but not having too much so that I felt shakey. Nothing really got rid of the inner tiredness though and I was a little concerned for my own safety, my reactions definitely weren't as sharp as usual.
Also as the ride went on I found it harder to consume real food. I'd been very sure to stay well hydrated, as evidenced by regular weeing and good urine colour, so it wasn't that. For the last 2 hours I just had energy gels (that I save for emergencies) as that's all I could get down.
Arrived home at 3am, 22 hours after setting off, exhausted but delighted to be back home and in one piece.
I barely slept that night and felt rotten for all of the next day. Fine today though after a good night's sleep.