Check out a video from Global Cycling Network about the important things that you should bring for a long bike ride so that you are prepared for every eventuality.
Since you can't bring everything with you during a bike ride due to limited space of your cycling jersey pockets and saddlebag. Also, if you bring unnecessary stuff it will just add additional load. The video will talk about what you should bring and where to put it.
Important things to bring are two inner tubes, a puncture repair kit, multi tool, mobile phone in a waterproof bag, raincape, a mini pump, your wallet, and an ID with your emergency contact number.