There can be a big difference between the theoretical amount of time a certain amount of kilometres should take, and the actual time. Hard to explain...In rural and semi-rural areas where you can calculate the distance between Nowheresville and Hicktown the distance might be 100 kilometres, say. Average speed of 22km per hour. A few quick stops inbetween. It will take five hours. Impossible to get lost as you're just following the road through the main streets of a few small towns.

There are many variables though. Wind, a flat tyre, taking the wrong road, road surface etc. You might feel like riding faster that day; perhaps slower. Where a cycle tourist can run into trouble is getting through bigger cities. Especially if the signs are in a foreign language or designed for locals and are illogical. I spent half a day trying to get through Hangzhou in China because the city is full of bridges & elevated roads, and most are off-limits to bicycles. Maps do not provide this information

A guesstimate of the total distance isn't hard to determine nowdays using Google Maps. The time taken to cover that route is more subjective. I calculate the total distance, divide by the number of kilometres per day I expect to travel, then allow a handy margin of error to cover navigation errors, mechanical errors and fatigue.