Well done, rock_ten... I mean it.

I hope you learned something from the effort. The neck soreness is probably to do more with bike fit than anything else, but you can tune that in as you go along. More serious cases of it occur on the longest randonnees, and it's called Shermer's Neck after the RAAM rider who gained most notoriety from suffering it. In the worst cases, the rider loses all ability to keep the head upright.

Your ailments afterwards (stiffness, headache) all sound like dehydration issues (particularly as you drank a lot at the finish). This is something you really need to keep an eye on if you continue doing long distances. Riding in cool to cold weather tends to hide the need to drink regularly.

The cramping, high heart rate and your listlessness in sleeping are indicators of this. High heart rate also occurs after a long period of exercise like this, but should settle right down within 24 hours. If not... be cautious and see a doctor.

The emotional lows also can be linked to inadequate intake of food. But you've already identified that.

Probably the best investment you made in the whole adventure was the Brooks saddle.