personally I am a fan of breaks, good for my muscles, I will stretch, snack, take off my bike shoes, whatever.
Honestly, I dont keep track of breaks, but I suspect that I will take one every hour or less, but for how long, how often really depends on how I am feeling that day and how the conditions are (headwind, climbing etc)

as Ive mentioned before, dont take 100k as a given, especially at teh beginning of a trip when you arent as strong. Set lower limits and gradually increase your days. As for your specific question, you can get guidelines from what others say, but you really have to listen to yourself about wanting a breather, needing food or whatever.

If you can do loaded test rides, you will quickly get an idea of what works for you and what speed is realistic (factoring in the "latter part of the day tiredness" too)
20kph is most likely too high, mine has been usually around 17 averaged out.