Cycling is very different from long distance running in that it's a low-impact activity. In marathon training, you try to ramp up gradually to the event and you don't try to run 42k before the race because there is a high risk of injuries if you do otherwise. In cycling, that's generally not a problem. If you have a properly fitted bicycle and you're accustomed to sitting on it for several hours straight without major discomfort, it is not difficult to increase your distance quickly to 100 km or more, just using your body as a guide.

Try to get to 100 km as soon as possible and ride that distance every weekend. On top of that, do at least two 1-2 hour intensive rides during the week.