My perspective on training your body for long rides is time/distance and last, speed. Time is first because you're only going to ride as long and as far as your butt will let you. Make sure you have a saddle and a bike fit that allows you to do this, and comfortable shorts. The distance goes hand in hand with this, because if you are doing the miles you will rack up the time. A good guideline I once heard is that you can do a distance in one day when you can do 3 consecutive days of riding of shorter distances that add up to that single day's goal. So if your goal is 100 km, if you can do 3 days in a row of at least 33 km & not hurt too bad, you should be ok. Finally, speed does matter. If you can build up speed, you will be able to minimize the time it takes you to go the distance. It all works together.