PHONE (fully charged)
UNDER-SADDLE BAG, sometimes called a tube bag but you should cram in:
A pair of tyre levers.
Two spare inners tubes (valves the correct length for your rims).
A few small-size cable ties (they weigh nothing and can be surprisingly useful).
FUEL. Enough to get you to the first feed station (where re-stock) e.g., energy bars, gels, but consider old fashioned food too. Bananas remain hard to beat.
TWO BOTTLES. (500 ml size is sufficient for our climate, though 750ml is now the standard. NOTE: If you find you have been carrying almost a litre and a half of liquid (1.5 kilos) for most of your ride, then you have not planned your drinking/refilling) very well.
A BREATHABLE RAIN TOP - that's if the weather forecast is for moderate rain or worse. If your top layer is not made of breathable fabric with vents you will just get wet on the inside and then cold.