Ah, that makes sense, training is exercise with a purpose!
You need to do some shorter fast rides and some long slow rides (dead easy!).
The shorter faster rides will imporve your 'speed' so that you can do the 100 miles (or km) quicker.
The longer rides will improve your endurance.
For the shorter rides try 45 mins to 1 h; with a 10 min warm up and 10 min cool down. Ride the bit in the middle as fast as you can manage. On another day do 45 minutes to 1 h at a steady comfortable pace.
For the longer rides, start at a distance you are able to do and add on equal amounts (say 2 miles (or kms) a week) until you get to 75 miles (or kms) once you can do 75 miles or kms you'll easily manage the 100 miles or kms! Time your rides, and if the weather is bad or you feel rough and don't go your target distance just go home (no matter what anyone tells you miles on the road are really not that important (do the hours)).
To make life more interesting as you get a bit fitter, start looking for some hills and have some sprints for signs. Anything to get your heart and legs pounding.
Start off with three rides a week and build up to 5 or 6 only when you are fit enough to handle it!
For the longer rides you will need to think about carbohydrate replacement and hydration. If you are overweight, losing a few inches will help as well.