Well that was an interesting experience. Did my first 50mi ride today.
First 30 miles were ok though I didn't realise just how much the wind took out of me.
The next four miles were into a very stong headwind and being a cold wind I lost all core temperature. Normally I'd just spin a bit faster for a few mins to get the heartrate up but my legs didn't want to know.
At about 35 miles my rear mudguard fell apart resulting in a 5 min delay going through a hedgerow trying to find it.
At 45 miles my body gave up completely and I though i'd lost the ability to see properly, though a quick stop revealed I'd actually lost a contact lens.

Trying to get the other one out with frozen hands was fun!
Conclusions:
I need to take on more fuel pre ride. I intentionally didn't take much ( just normal breakfast) as I want to experiment with different options and needed a baseline, though I think the wind scuppered that plan.
I do a lot of running and cycling and running seem to differ when reaching the limit. When running my performance drops of gradually. With cycling it seems more like fine one minute gone the next.