Looking back in the light of what people are saying I think the primary issue was electrolytes, coupled with the fact it was the furthest I'd done solo and the furthest I'd done at any meaningful speed. I've done 80+ miles in a day before but always at a much more leisurely pace. So a need to improve my fitness will also be in there somewhere too.

Looks like more longer rides are needed... I know I'd need to do more rides but was surprised at the way my legs were responding to the hills.