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The last 10-15 miles will feel like death, hills you'd normally bomb up will level you to a crawl by that point, just make sure you keep pedalling, it stops lactic acid from building up. Don't forget your motivation, towards the end will get sticky but keep chugging. Remember to eat and drink at regular intervals (even if your not hungry/ thirsty) Most importantly, keep the pace consistent, don't go bombing at your normal speed for the first 20-40 miles or you WILL feel it towards the end. Carry some basic tools, just an allen key and a puncture repair kit, with you.
I wouldn't really worry too much about the other people, even if your not talking to each other, having others near by serves as a good motivator. Just try not to get swept up in their pace.
Anyway, good luck anon, riding a century is a big milestone!