To complete a century, I remember 2 things. Just keep pedalling. Speed isn't my goal; finishing is.
To ride a fast century actually requires training - at least for me. Fast is relative, but for me that would be averaging 16mph including off-bike time.
That being said, I carry more liquid than most people I ride with. I drink more and likely eat more, snacking semi-constantly.
I find the more stops I make, the slower my on-bike speed averages. There are obvious exceptions - like the rides when heat just gets to me. So, I remember somethnig I read here - even walking is faster than the speed at any stop - and try to keep moving. Learning to snack while riding has helped with this a lot. Now I've got my average number of stops down to 4 per 100 mile route.
Pre-ride bike maintenance is a must. An ounce of prevention and all that. Post-ride maintenance, or at least making notes of things to take care of before the next ride, also helps ensure success.