Start working out on a daily basis. You will need to be in great condition to complete this challenge. If you are not a member of a fitness center, you need to be. Start walking and lifting weights on a daily basis to build your stamina.
Ask a trainer at the fitness center to help you get in shape for your goal. Once you do this, you will have someone to encourage and push you through the hard times and support you until you complete the 100 miles.
Bike daily. This can be done on a stationary bike at the fitness center but you should try to ride the bike you will be going 100 miles on. Find a bike trail in your area and ride as much as possible.
Stay focused. Times are going to get really hard for you during this training process but if you have your mind set on this goal, do not let anything change your mind.
Enter bike races and charity riding events in your area. Most of these events are 25-50 miles but they will be perfect practice for you. Compete in as many as possible before your big ride.