If you can do 50 miles, then you can do 100. It is not that much harder. You just have to make one or two bathroom/water/snack stops then just keep on pedalling.

Don't think about the distance much. Just enjoy the ride. I like to look for wildlife. On one ride I saw a lot of ducks, geese, a three deer swimming in some flood waters and a couple of cranes.

Also, for long rides, you don't want to push yourself too hard. If you are so winded that you can't talk, you are pushing yourself too hard and will be worn out in about another 10 miles.