What to carry

A mobile phone.

A small under-saddle bag, sometimes called a tube bag but often you can cram in more.
A multi-tool.
A pair of tyre levers.
A pair of spare inners. (maybe also a puncture repair kit).
A few small-size cable ties (you'd be amazed)

Enough fuel (solid/liquid form) to get you to the first feed station (where re-stock)
Consider bars, sachets, but don't overlook old style food, eg. banana.

2 bottles. (500 ml size is sufficient for our climate).

Depending on weather forecast, a breathable rain top.